Obd2 diagnostics

obd2,diagnostics

1. S
ome scan tools call it the
global OBD II mode, while
others describe it as the
OBD II generic mode. The
OBD II generic mode allows
a technician to attach his
scan tool to an OBD II-compliant vehi-
cle and begin collecting data without
entering any VIN information into the
scan tool. You may need to specifically
select “OBD II Generic” from the scan
tool menu. Some scan tools may need a
software module or personality key be-
fore they’ll work in generic OBD II test
mode.
The original list of generic data pa-
rameters mandated by OBD II and de-
scribed in SAE J1979 was short and de-
signed to provide critical system data
only. The useful types of data we can re-
trieve from OBD II generic include
short-term and long-term fuel trim val- ues, oxygen sensor voltages, engine and intake air temperatures, MAF or MAP values, rpm, calculated load, spark tim- ing and diagnostic trouble code (DTC) count. Freeze frame data and readiness status also are available in OBD II generic mode. A generic scan tool also should be able to erase trouble codes and freeze frame data when command- ed to do so.
Data coming to the scan tool through
the mandated OBD II generic interface
may not arrive as fast as data sent over
one of the dedicated data link connec-
tor (DLC) terminals. The vehicle man-
ufacturer has the option of using a
faster data transfer speed on other DLC
pins. Data on the generic interface also
may not be as complete as the informa-
tion you’ll get on many manufacturer-
52 September 2007
OBDII
GENERIC
PID
DIAGNOSIS
BYKARLSEYFERT
A wealth of diagnostic information is
available on late-model OBD II-compliant
vehicles, even when ‘enhanced’ or
‘manufacturer-specific’ PIDs are not
accessible. It doesn’t take much to use
this information to its best advantage.
Photo: Karl Seyfert

2. 53September 2007

3. specific or enhanced interfaces. For ex-
ample, you may see an engine coolant
temperature (ECT) value in degrees on
the OBD II generic parameter identi-
fication (PID) list. A manufacturer-
specific data list may display ECT status
in Fahrenheit or Celsius and add a sep-
arate PID for the ECT signal voltage.
In spite of these and other limitations,
OBD II generic mode still contains
many of the trouble codes, freeze frame
data and basic datastream information
needed to solve many emissions-related
issues.
There are nine modes of operation
described in the original J1979 OBD II
standard. They are:
Mode 1: Show current data
Mode 2: Show freeze frame data
Mode 3: Show stored trouble codes
Mode 4: Clear trouble codes and stored
values
Mode 5: Test results, oxygen sensors
Mode 6: Test results, noncontinuously
monitored
Mode 7: Show pending trouble codes
Mode 8: Special control mode
Mode 9: Request vehicle information
Modes 1 and 2 are basically identical.
Mode 1 provides current information,
Mode 2 a snapshot of the same data
taken at the point when the last diag-
nostic trouble code was set. The excep-
tions are PID 01, which is available only
in Mode 1, and PID 02, available only
in Mode 2. If Mode 2 PID 02 returns
zero, then there’s no snapshot and all
other Mode 2 data is meaningless. Vehi-
cle manufacturers are not required to
support all modes. Each manufacturer
may define additional modes above
Mode 9 for other information.
Most vehicles from the J1979 era sup-
ported 13 to 20 parameters. The recent
phase-in of new parameters will make
OBD II generic data even more valu-
able. The California Air Resources
Board (CARB) revisions to OBD II
CAN-equipped vehicles have increased
the number of potential generic param-
eters to more than a hundred. Not all
vehicles will support all PIDs, and there
are many manufacturer-defined PIDs
that are not included in the OBD II
standard. Even so, the quality and
quantity of data have increased signifi-
cantly. For more information on the new
PIDs that were added to 2004 and later
CAN-equipped vehicles, refer to Bob
Pattengale’s article “Interpreting Gener-
ic Scan Data” in the March 2005 issue of
M
OTOR. A PDF copy of the article can
be downloaded at www.motor.com.
Establish a Baseline
If you’re repairing a vehicle that has
stored one or more DTCs, make sure
you collect the freeze frame data before
erasing the stored codes. This data can
54 September 2007
OBD II GENERIC PID DIAGNOSIS
Here’s a basic scanner display showing OBD II generic
PIDs. Slow-changing PIDs like IAT and ECT can be fol-
lowed fairly easily in this format, but it’s difficult to spot
glitches in faster moving PIDs like Spark Advance. This scan tool also allows the user to graph some PIDs,
while continuing to display the others in conventional
numeric format. Due to OBD II’s refresh capabilities on
some vehicles, it’s best to limit your PID choices to
those directly related to your diagnostic approach.
This photo illustrates how far PID data collection and display have come. Several
hundred thousand techs are still using the original Snap-on “brick” (on the left),
which displays a limited amount of PID data on its screen. Scrolling up or down
revealed more PIDs. The color version on the right brought graphing capability to
the brick, and extended the product’s life span by several years.
Photos: Karl Seyfert
Photo courtesy Snap-on Diagnostics

4. be used for comparison after
your repairs. The “before”
freeze frame shot and its PID
data establish the baseline.
As you begin your diagno-
sis, correct basic problems
first—loose belts, weak bat-
teries, corroded cables, low
coolant levels and the like.
The battery and charging sys-
tem are especially important,
due to their effect on vehicle
electronics. A good battery, a
properly functioning alterna-
tor and good connections at
power and ground circuits
are essential. You can’t as-
sume that OBD II will detect
a voltage supply problem that
can affect the entire system.
If you have an intermittent
problem that comes and
goes, or random problems
that don’t follow a logical pattern, check
the grounds for the PCM and any other
controller in the vehicle.
If the basics check out, focus your di-
agnosis on critical engine parameters
and sensors first. Write down what you
find; there’s too much information to
keep it all in your head. Add any infor-
mation collected from the vehicle own-
er regarding vehicle performance. Jot
down the battery voltage and the results
of any simple tests, such as fuel pressure
or engine vacuum. Look at the Readi-
ness Status display to see if there are
any monitors that aren’t running to
completion.
Datastream Analysis
Take your time when you begin looking
at the live OBD II datastream. If you se-
lect too many items at one time, the scan
tool update will slow. The more PIDs
you select, the slower the update rate
will be. Look carefully at the PIDs and
their values. Is there one line of data that
seems wrong? Compare data items to
one another.
Do MAP and BARO agree key on,
engine off (KOEO)? Are IAT and ECT
the same when the engine is cold
KOEO? The ECT and IAT should be
within 5°F of each other. ECT should
reach operating temperature, preferably
190°F or higher. If the ECT is too low,
the PCM may richen the fuel mixture to
compensate for a (perceived) cold-
engine condition. IAT should read ambi-
ent temperature or close to underhood
temperature, depending on the location
of the sensor.
Is the battery voltage good KOEO?
Is the charging voltage adequate when
the engine starts? Do the MAP and
BARO readings seem logical? Do the
IAC counts look too high or
too low? Compare data items
to known-good values you’d
expect to see for similar op-
erating conditions on similar
vehicles.
Check short-term fuel
trim (STFT) and long-term
fuel trim (LTFT). Fuel trim
is a key diagnostic parameter
and tells you what the com-
puter is doing to control fuel
delivery and how the adap-
tive strategy is operating.
STFT and LTFT are ex-
pressed as a percentage, with
the ideal range being within
±5%. Positive fuel trim per-
centages indicate that the
powertrain control module
(PCM) is attempting to en-
richen the fuel mixture to
compensate for a perceived
lean condition. Negative fuel trim per-
centages indicate that the PCM is at-
tempting to enlean the fuel mixture to
compensate for a perceived rich condi-
tion. STFT will normally sweep rapidly
between enrichment and enleanment,
while LTFT will remain more stable. If
either STFT or LTFT exceeds ±10%,
this should alert you to a potential
problem.
56 September 2007
OBD II GENERIC PID DIAGNOSIS
The Snap-on MODIS is a combination scanner, lab/igni-
tion scope, DVOM and Troubleshooter. In scanner mode,
MODIS can graph several parameters simultaneously,
as seen in this screen capture. Remember, although
these may look like scope patterns, the reporting rate
for PID data on a scanner isn’t nearly as fast.
When scan tool screen real estate is
limited, porting the scan tool into a
laptop or desktop PC allows you to
graph more PIDs simultaneously.
The PC’s much larger memory ca-
pacity also makes it possible to col-
lect PID data in movie format for
later playback and analysis.
An on-screen description of the PID
displayed below the graphing data
may help you to understand what
you’re looking at, and avoid misunder-
standings with measurement units.
Screen capture: Jorge Menchu
Photo courtesy SPX/OTC
Photo courtesy Injectoclean

5. Determine if the condition exists in
more than one operating range. Check
fuel trim at idle, at 1500 rpm and at
2500 rpm. If LTFT B1 is 20% at idle but
corrects to 5% at both 1500 and 2500
rpm, focus your diagnosis on factors that
can cause a lean condition at idle, such
as a vacuum leak. If the condition exists
in all rpm ranges, the cause is more like-
ly to be fuel-related, such as a bad fuel
pump, restricted injectors, etc.
Fuel trim can also be used to identify
which bank of cylinders is causing a
problem on bank-to-bank fuel control
engines. For example, if LTFT B1 is
25% and LTFT B2 is 5%, the source
of the problem is associated with B1
cylinders only, and your diagnosis
should focus on factors related to B1
cylinders only.
The following parameters could af-
fect fuel trim or provide additional diag-
nostic information. Also, even if fuel
trim is not a concern, you might find an
indication of another problem when re-
viewing these parameters:
Fuel System 1 Status and Fuel Sys-
tem 2 Status should be in closed-loop
(CL). If the PCM is not able to
achieve CL, the fuel trim data may not
be accurate.
If the system includes one, the mass
airflow (MAF) sensor measures the
amount of air flowing into the engine.
The PCM uses this information to cal-
culate the amount of fuel that should be
delivered to achieve the desired air/fuel
mixture. Check the MAF sensor for ac-
curacy in various rpm ranges, including
wide-open throttle (WOT), and com-
pare it with the manufacturer’s recom-
mendations.
When checking MAF sensor read-
ings, be sure to identify the unit of mea-
surement. The scan tool may report the
information in grams per second (gm/S)
or pounds per minute (lb/min). Some
technicians replace the sensor, only to
realize later that the scan tool was not
set correctly. Some scan tools let you
change the units of measurement for
different PIDs so the scan tool matches
the specification in your reference man-
ual. Most scan tools let you switch easily
between Fahrenheit and Celsius tem-
perature scales, for example. But MAF
specs can be confusing when the scan
tool shows lb/min and we have a spec
for gm/S. Here are a few common con-
version formulas, in case your scan tool
doesn’t support all of these units of
measurement:
Degrees Fahrenheit 32 5/9 Degrees Celsius
Degrees Celsius 9/5 + 32 Degrees Fahrenheit
lb/min 7.5 gm/S
gm/S 1.32 lb/min
The Manifold Absolute Pressure
(MAP) Sensor PID, if available, indi-
cates manifold pressure, which is used
by the PCM to calculate engine load.
The reading is normally displayed in
inches of mercury (in./Hg). Don’t con-
fuse the MAP sensor parameter with in-
take manifold vacuum; they’re not the
same. Use this formula: barometric
58 September 2007
OBD II GENERIC PID DIAGNOSIS
Graphs aren’t the only way to display PID data. Once
transferred to the PC with its greater screen real es-
tate, PID data can be converted to formats that relate
to the data. A red thermometer scale is much easier to
follow than changing numbers on a scan tool.
PC-based scan tools excel at capturing and displaying
large amounts of PID data for later analysis. Graphing
the data, then analyzing it on-screen, may allow you to
spot inconsistencies and provides an easy method for
overlaying similar or related PID data.
Here’s a peek at some of the addition-
al PID data that’s available on late-
model vehicles. This screen capture
was taken from a CAN-enabled 2005
vehicle, and includes PIDs for EVAP
PURGE, FUEL LEVEL and WARM-UPS, as
well as familiar PIDs like BARO. This
much PID data in generic mode should
aid in diagnosis when manufacturer-
specific PID data is not available.
Screen captures: Jorge Menchu
Screen capture courtesy Bosch Diagnostics

6. pressure (BARO) MAP intake
manifold vacuum. For example, BARO
(27.5 in./Hg) MAP (10.5) intake
manifold vacuum (17.0 in./Hg). Some
vehicles are equipped with only a MAF
sensor, some have only a MAP sensor
and some are equipped with both.
The PIDs for Oxygen Sensor Output
Voltage B1S1, B2S1, B1S2, etc., are
used by the PCM to control fuel mix-
ture and to detect catalytic converter
degradation. The scan tool can be used
to check basic sensor operation. The
sensor must exceed .8 volt and drop be-
low .2 volt, and the transition from low
to high and high to low should be quick.
A good snap throttle test will verify the
sensor’s ability to achieve the .8 and .2
voltage limits. If this method doesn’t
work, use a bottle of propane to manu-
ally richen the fuel mixture to check the
oxygen sensor’s maximum voltage out-
put. To check the sensor’s low voltage
range, simply create a lean condition
and check the voltage.
Remember, your scan tool is not a lab
scope. You’re not measuring the sensor
in real time. The PCM receives the data
from the oxygen sensor, processes it,
then reports it to the scan tool. Also, a
fundamental OBD II generic limitation
is the speed at which that data is deliv-
ered to the scan tool. In most cases, the
fastest possible data rate is approximate-
ly 10 times a second, with only one pa-
rameter selected. If you’re requesting
and/or displaying 10 parameters, this
slows the data sample rate, and each pa-
rameter is reported to the scan tool just
once per second. You can achieve the
best results by graphing or displaying
data from each oxygen sensor separately.
If the transition seems slow, the sensor
should be tested with a lab scope to veri-
fy the diagnosis before you replace it.
The Engine Speed (RPM) and Igni-
tion Timing Advance PIDs can be used
to verify good idle control strategy.
Again, these are best checked using a
graphing scan tool. Check the RPM,
Vehicle Speed Sensor (VSS) and Throt-
tle Position Sensor (TPS) PIDs for ac-
curacy. These parameters can also be
used as reference points to duplicate
symptoms and locate problems in
recordings.
Most PID values can be verified by
a voltage, frequency, temperature,
vacuum or pressure test. Engine
coolant temperature, for example, can
be verified with a noncontact temper-
ature tester, while intake manifold vac-
uum can be verified with an accurate
vacuum gauge. Electrical values also
should be tested with a DVOM. If the
electrical value exists at the sensor but
not at the appropriate PCM terminal,
then the component might be experi-
encing a circuit fault.
Calculated Values
Calculated scan tool values can cause a
lot of confusion. The PCM may detect a
failed ECT sensor or circuit and store a
DTC. Without the ECT sensor input,
59September 2007
Circle #31

7. the PCM has no idea what the coolant
temperature really is, so it may “plug in”
a temperature it thinks will work to
keep the engine running long enough to
get it to a repair shop. When it does
this, your scanner will display the fail-
safe value. You might think it’s a live val-
ue from a working sensor, when it isn’t.
Also be aware that when a compo-
nent such as an oxygen sensor is discon-
nected, the PCM may substitute a de-
fault value into the datastream displayed
on the scan tool. If a PID is static and
doesn’t track with engine operating con-
ditions, it may be a default value that
merits further investigation. Graphing Data
If you’ve ever found it difficult to com-
pare several parameters at once on a
small scan tool screen, graphing PIDs is
an appealing proposition. Graphing
multiple parameters at the same time
can help you compare data and look for
individual signals that don’t match up to
actual operating conditions.
Although scan tool graphing isn’t
equivalent in quality and accuracy to a
lab scope reading, it can provide a com-
parative analysis of the activity in the
two, three, four or six oxygen sensors
found in most OBD II systems.
Many scan tools are capable of stor-
ing a multiple-frame movie of selected
PIDs. The scan tool can be programmed
to record a movie after a specific DTC
is stored in the PCM. Alternatively, the
scan tool movie might be triggered
manually when a driveability symptom
occurs. In either case, you can observe
the data or download it and print it lat-
er. Several software programs let you
download a movie, then plot the values
in a graphical display on your computer
monitor.
Make the Most of
What You’ve Got
Take the time to learn what your scan
tool will do when connected to a spe-
cific make or model. Do your best to
gather all relevant information about
the vehicle system being tested. That
way you can get the most out of what
the scan tool and PCM have to offer.
The OBD II system won’t store a DTC
unless it sees (or thinks it sees) a prob-
lem that can result in increased emis-
sions. The only way to know what the
PCM sees (or thinks it sees) is to look
through the window provided by the
scan tool interface.
You have a DTC and its definition.
You have freeze frame data that may
help you zero in on the affected compo-
nent or subsystem. PIDs have already
provided you with additional clues
about the operation of critical sensors.
Keep your diagnosis simple as long as
you can. Now fix the car.
60 September 2007
OBD II GENERIC PID DIAGNOSIS
Visit www.motor.comto download
a free copy of this article.
Circle #32
Circle #33
Circle #34
Circle #35

8. Photo & screen captures: Bob Pattengale
52 March 2005

9. I
f you don’t have a good starting point, driveability diagnostics
can be a frustrating experience. One of the best places to start
is with a scan tool. The question asked by many is, “Which
scan tool should I use?” In a perfect world with unlimited re-
sources, the first choice would probably be the factory scan tool.
Unfortunately, most technicians
don’t have extra-deep pockets. That’s
why my first choice is an OBD II
generic scan tool. I’ve found that ap-
proximately 80% of the driveability
problems I diagnose can be narrowed
down or solved using nothing more
than OBD II generic parameters. And
all of that information is available on
an OBD II generic scan tool that can
be purchased for under $300.
The good news is the recent phase-in
of new parameters will make OBD II
generic data even more valuable. Fig. 1
on page 54 was taken from a 2002 Nis-
san Maxima and shows the typical para-
meters available on most OBD II-
equipped vehicles. As many as 36 para-
meters were available under the original
OBD II specification. Most vehicles
from that era will support 13 to 20 para-
meters. The California Air Resources
Board (CARB) revisions to OBD II
CAN-equipped vehicles will increase
the number of potential generic para-
meters to more than 100. Fig. 2 on page
56 shows data from a CAN-equipped
2005 Dodge Durango. As you can see,
the quality and quantity of data has in-
creased significantly. This article will
identify the parameters that provide the
greatest amount of useful information
and take a look at the new parameters
that are being phased in.
No matter what the driveability is-
sue happens to be, the first parame-
53March 2005
INTERPRETING
GENERIC
SCAN DATA
BYBOBPATTENGALE
Readily available ‘generic’ scan data provides an
excellent foundation for OBD II diagnostics.
Recent enhancements have increased the value of
this information when servicing newer vehicles.

10. ters to check are short-term fuel trim
(STFT) and long-term fuel trim
(LTFT). Fuel trim is a key diagnostic
parameter and your window into what
the computer is doing to control fuel
delivery and how the adaptive strategy
is operating. STFT and LTFT are ex-
pressed as a percentage, with the ideal
range being within 5%. Positive fuel
trim percentages indicate that the
powertrain control module (PCM) is
attempting to enrichen the fuel mix-
ture to compensate for a perceived
lean condition. Negative fuel trim
percentages indicate that the PCM is
attempting to enlean the fuel mixture
to compensate for a perceived rich
condition. STFT will normally sweep
rapidly between enrichment and en-
leanment, while LTFT will remain
more stable. If STFT or LTFT ex-
ceeds 10%, this should alert you to
a potential problem.
The next step is to determine if the
condition exists in more than one op-
erating range. Fuel trim should be
checked at idle, at 1500 rpm and at
2500 rpm. For example, if LTFT B1 is
25% at idle but corrects to 4% at both
1500 and 2500 rpm, your diagnosis
should focus on factors that can cause
a lean condition at idle, such as a vacu-
um leak. If the condition exists in all
rpm ranges, the cause is more likely to
be fuel supply-related, such as a bad
fuel pump, restricted injectors, etc.
Fuel trim can also be used to identi-
fy which bank of cylinders is causing a
problem. This will work only on bank-
to-bank fuel control engines. For ex-
ample, if LTFT B1 is 20% and LTFT
B2 is 3%, the source of the problem is
associated with B1 cylinders only, and
your diagnosis should focus on factors
related to B1 cylinders only.
The following parameters could af-
fect fuel trim or provide additional
diagnostic information. Also, even if
fuel trim is not a concern, you might
find an indication of another problem
when reviewing these parameters:
Fuel System 1 Status andFuel
System 2 Statusshould be in closed-
loop (CL). If the PCM is not able to
achieve CL, the fuel trim data may not
be accurate.
Engine Coolant Temperature
(ECT)should reach operating temper-
ature, preferably 190°F or higher. If
the ECT is too low, the PCM may
richen the fuel mixture to compensate
for a (perceived) cold engine condition.
Intake Air Temperature (IAT)
should read ambient temperature or
close to underhood temperature, de-
pending on the location of the sensor.
In the case of a cold engine check—
Key On Engine Off (KOEO)—the
ECT and IAT should be within 5°F of
each other.
The Mass Airflow (MAF)Sensor,
if the system includes one, measures
the amount of air flowing into the en-
gine. The PCM uses this information
to calculate the amount of fuel that
54 March 2005
INTERPRETING GENERIC SCAN DATA
Fig. 1

11. should be delivered, to achieve the
desired air/fuel mixture. The MAF
sensor should be checked for accura-
cy in various rpm ranges, including
wide-open throttle (WOT), and com-
pared with the manufacturer’s recom-
mendations. Mark Warren’s Dec.
2003 Driveability Corner column cov-
ered volumetric efficiency, which
should help you with MAF diagnos-
tics. A copy of that article is available
at www.motor.com, and an updated
volumetric efficiency chart is available
at www.pwrtraining.com.
When checking MAF sensor read-
ings, be sure to identify the unit of
measurement. The scan tool may re-
port the information in grams per sec-
ond (gm/S) or pounds per minute
(lb/min). For example, if the MAF
sensor specification is 4 to 6 gm/S and
your scan tool is reporting .6 lb/min,
change from English units to metric
units to obtain accurate readings.
Some technicians replace the sensor,
only to realize later that the scan tool
was not set correctly. The scan tool
manufacturer might display the para-
meter in both gm/S and lb/min to help
avoid this confusion.
The Manifold Absolute Pressure
(MAP)Sensor, if available, measures
manifold pressure, which is used by
the PCM to calculate engine load. The
reading in English units is normally
displayed in inches of mercury
(in./Hg). Don’t confuse the MAP sen-
sor parameter with intake manifold
vacuum; they’re not the same. A sim-
ple formula to use is: barometric pres-
sure (BARO) MAP intake mani-
fold vacuum. For example, BARO
27.5 in./Hg MAP 10.5 intake
manifold vacuum of 17.0 in./Hg. Some
vehicles are equipped with only a
MAF sensor, some have only a MAP
sensor and some are equipped with
both sensors.
Oxygen Sensor Output Voltage
B1S1, B2S1, B1S2, etc., are used by
the PCM to control fuel mixture. An-
other use for the oxygen sensors is to
detect catalytic converter degradation.
The scan tool can be used to check ba-
sic sensor operation. Another way to
test oxygen sensors is with a graphing
scan tool, but you can still use the data
grid if graphing is not available on
your scanner. Most scan tools on the
market now have some form of graph-
ing capability.
The process for testing the sensors
is simple: The sensor needs to exceed
.8 volt and drop below .2 volt, and the
transition from low to high and high
to low should be quick. In most cases,
a good snap throttle test will verify
the sensor’s ability to achieve the .8
and .2 voltage limits. If this method
does not work, use a bottle of
propane to manually richen the fuel
mixture to check the oxygen sensor’s
maximum output. To check the low
oxygen sensor range, simply create a
lean condition and check the voltage.
Checking oxygen sensor speed is
where a graphing scan tool helps. Fig.
3 on page 57 and Fig. 4 on page 58
show examples of oxygen sensor data
graphed, along with STFT, LTFT and
rpm, taken from two different graph-
ing scan tools.
Remember, your scan tool is not a
lab scope. You’re not measuring the
56 March 2005
INTERPRETING GENERIC SCAN DATA
Fig. 2

12. sensor in real time. The PCM re-
ceives the data from the oxygen sen-
sor, processes it, then reports it to the
scan tool. Also, a fundamental OBD
II generic limitation is the speed at
which that data is delivered to the
scan tool. In most cases, the fastest
possible data rate is approximately 10
times a second with only one parame-
ter selected. If you’re requesting
and/or displaying 10 parameters, this
slows the data sample rate, and each
parameter is reported to the scan tool
just once per second. You can achieve
the best results by graphing or dis-
playing data from each oxygen sensor
separately. If the transition seems
slow, the sensor should be tested with
a lab scope to verify the diagnosis be-
fore you replace it.
Engine Speed (RPM)and Igni-
tion Timing Advancecan be used
to verify good idle control strategy.
Again, these are best checked using a
graphing scan tool.
The RPM, Vehicle Speed Sensor
(VSS)and Throttle Position Sensor
(TPS)should be checked for accuracy.
These parameters can also be used as
reference points to duplicate symptoms
and locate problems in recordings.
Calculated Load, MIL Status,
Fuel Pressure andAuxiliary Input
Status (PTO)should also be consid-
ered, if they are reported.
Additional OBD II
Parameters
Now, let’s take a look at the more re-
cently introduced OBD II parameters.
These parameters were added on 2004
CAN-equipped vehicles, but may also
be found on earlier models or non-
CAN-equipped vehicles. For example,
the air/fuel sensor parameters were
available on earlier Toyota OBD II ve-
hicles. Fig. 2 was taken from a 2005
Dodge Durango and shows many of
the new parameters. Parameter de-
scriptions from Fig. 2 are followed by
the general OBD II description:
FUEL STAT 1 Fuel System 1
Status:Fuel system status will display
more than just Closed Loop (CL) or
Open Loop (OL). You might find one
of the following messages: OL-Drive,
indicating an open-loop condition
during power enrichment or decelera-
tion enleanment; OL-Fault, indicating
the PCM is commanding open-loop
due to a system fault; CL-Fault, indi-
cating the PCM may be using a differ-
ent fuel control strategy due to an
oxygen sensor fault.
ENG RUN TIME Time Since En-
gineStart:This parameter may be
useful in determining when a particu-
lar problem occurs during an engine
run cycle.
DIST MIL ON Distance Traveled
While MIL Is Activated:This para-
meter can be very useful in determin-
ing how long the customer has al-
lowed a problem to exist.
COMMAND EGR EGR_PCT:
Commanded EGR is displayed as a
percentage and is normalized for all
EGR systems. EGR commanded
OFF or Closed will display 0%, and
EGR commanded to the fully open
57March 2005
Fig. 3

13. position will display 100%. Keep in
mind this parameter does not reflect
the quantity of EGR flow—only what
the PCM is commanding.
EGR ERROR EGR_ERR:This
parameter is displayed in percentage
and represents EGR position errors.
The EGR Error is also normalized for
all types of EGR systems. The reading
is based on a simple formula: (Actual
EGR Position Commanded EGR)
Commanded EGR EGR Error. For
example, if the EGR valve is command-
ed open 10% and the EGR valve moves
only 5% (5% 10%) 10% 50%
error. If the scan tool displays EGR Er-
ror at 99.2% and the EGR is command-
ed OFF, this indicates that the PCM is
receiving information that the EGR
valve position is greater than 0%. This
may be due to an EGR valve that is
stuck partially open or a malfunctioning
EGR position sensor.
EVAP PURGE EVAP_PCT: This
parameter is displayed as a percent-
age and is normalized for all types of
purge systems. EVAP Purge Control
commanded OFF will display 0% and
EVAP Purge Control commanded
fully open will display 100%. This is
an important parameter to check if
the vehicle is having fuel trim prob-
lems. Fuel trim readings may be ab-
normal, due to normal purge opera-
tion. To eliminate EVAP Purge as a
potential contributor to a fuel trim
problem, block the purge valve inlet
to the intake manifold, then recheck
fuel trim.
FUEL LEVEL FUEL_PCT:Fuel
level input is a very useful parameter
when you’re attempting to complete
system monitors and diagnose specif-
ic problems. For example, the misfire
monitor on a 1999 Ford F-150 re-
quires the fuel tank level to be
greater than 15%. If you’re attempt-
ing to duplicate a misfire condition by
monitoring misfire counts and the fuel
level is under 15%, the misfire moni-
tor may not run. This is also impor-
tant for the evaporative emissions
monitor, where many manufacturers
require the fuel level to be above
15% and below 85%.
WARM-UPS WARM_UPS:This
parameter will count the number of
warm-ups since the DTCs were cleared.
A warm-up is defined as the ECT rising
at least 40°F from engine starting tem-
perature, then reaching a minimum
temperature of 160°F. This parameter
will be useful in verifying warm-up cy-
cles, if you’re attempting to duplicate a
specific code that requires at least two
warm-up cycles for completion.
BARO BARO:This parameter is
useful for diagnosing issues with
MAP and MAF sensors. Check this
parameter KOEO for accuracy relat-
ed to your elevation.
CAT TMP B1S1/B2S1
CATEMP11,21, etc.:Catalyst tem-
perature displays the substrate temper-
ature for a specific catalyst. The tem-
perature value may be obtained directly
from a sensor or inferred using other
sensor inputs. This parameter should
have significant value when checking
catalyst operation or looking at reasons
for premature catalyst failure, say, due
to overheating.
58 March 2005
INTERPRETING GENERIC SCAN DATA
Fig. 4

14. CTRL MOD (V) VPWR: I was
surprised this parameter was not in-
cluded in the original OBD II specifi-
cation. Voltage supply to the PCM is
critical and is overlooked by many
technicians. The voltage displayed
should be close to the voltage present
at the battery. This parameter can be
used to look for low voltage supply is-
sues. Keep in mind there are other
voltage supplies to the PCM. The igni-
tion voltage supply is a common source
of driveability issues, but can still be
checked only with an enhanced scan
tool or by direct measurement.
ABSOLUT LOAD LOAD_ABS:
This parameter is the normalized value
of air mass per intake stroke displayed
as a percentage. Absolute load value
ranges from 0% to approximately 95%
for normally aspirated engines and 0%
to 400% for boosted engines. The infor-
mation is used to schedule spark and
EGR rates, and to determine the
pumping efficiency of the engine for di-
agnostic purposes.
OL EQ RATIO EQ_RAT:Com-
manded equivalence ratio is used to de-
termine the commanded air/fuel ratio
of the engine. For conventional oxygen
sensor vehicles, the scan tool should dis-
play 1.0 in closed-loop and the PCM-
commanded EQ ratio during open-
loop. Wide-range and linear oxygen
sensors will display the PCM-com-
manded EQ ratio in both open-loop
and closed-loop. To calculate the actual
A/F ratio being commanded, multiply
the stoichiometric A/F ratio by the EQ
ratio. For example, stoichiometric is a
14.64:1 ratio for gasoline. If the com-
manded EQ ratio is .95, the command-
ed A/F is 14.64 0.95 13.9 A/F.
TP-B ABS, APP-D, APP-E, COM-
MAND TAC:These parameters relate
to the throttle-by-wire system on the
2005 Dodge Durango of Fig. 2 and will
be useful for diagnosing issues with this
system. There are other throttle-by-wire
generic parameters available for differ-
ent types of systems on other vehicles.
There are other parameters of inter-
est, but they’re not displayed or avail-
able on this vehicle. Misfire data will be
available for individual cylinders, similar
to the information displayed on a GM
enhanced scan tool. Also, if available,
wide-range and linear air/fuel sensors
are reported per sensor in voltage or
milliamp (mA) measurements.
Fig. 5 above shows a screen capture
from the Vetronix MTS 3100 Mas-
tertech. The red circle highlights the
“greater than” symbol (>), indicating
that multiple ECU responses differ in
value for this parameter. The blue cir-
cle highlights the equal sign (=), indi-
cating that more than one ECU sup-
ports this parameter and similar values
have been received for this parameter.
Another possible symbol is the excla-
mation point (!), indicating that no re-
sponses have been received for this
parameter, although it should be sup-
ported. This information will be useful
in diagnosing problems with data on
the CAN bus.
As you can see, OBD II generic data
has come a long way, and the data can
be very useful in the diagnostic process.
The important thing is to take time to
check each parameter and determine
how they relate to one another.
If you haven’t already purchased an
OBD II generic scan tool, look for
one that can graph and record, if pos-
sible. The benefits will immediately
pay off. The new parameters will take
some time to sort out, but the diag-
nostic value will be significant. Keep
in mind that the OBD II generic
specification is not always followed to
the letter, so it’s important to check
the vehicle service information for
variations and specifications.
60 March 2005
INTERPRETING GENERIC SCAN DATA
Fig. 5
Visit www.motor.comto download
a free copy of this article.

15. 37August 2008
L
ast month’s installment
on datastream analysis
focused on the value of
freeze frame data, Mode
5 and Mode 6 data and
KOEO (key on, engine
off) datastream. This month’s discussion
picks up where we left off, with KOER
(key on, engine running) analysis. So go
ahead, start the engine!
I recommend that KOER data col-
lection always start in the generic, or
global OBD II interface. Why? Because
generic datastream PID values are nev-
er substitutes for actual sensor readings.
For example, you can disconnect the
MAP sensor connector on a Chrysler
product and drive it around while moni-
toring datastream in the enhanced
(manufacturer-specific) interface. (Try
this yourself; don’t just take my word for
it.) You’ll see the MAP PID change
along with the TPS sensor reading and
rpm, showing a range of values that re-
flect likely MAP readings for each con-
dition, moment by moment. These are
substituted values. If you looked at the
MAP voltagePID, however, it would
show an unchanging reference voltage.
In the enhanced interface, substitutions
can and do occur. But in the generic in-
terface, substituted values are never al-
lowed. You would see MAP shown at a
constant pressure equal to something aDATASTREAM
IN-DEPTH
ANALYSIS
BYSAMBELL
We began this two-part article with a
discussion of preliminary OBD II datastream
analysis, conducted with the engine off.
We’re going deeper this time, to explain
the value of datastream information
collected with the engine running.
Photoillustration: Harold Perry; photos: Wieck Media & Jupiter Images

16. bit higher than BARO. The generic in-
terface allows calculatedvalues, but
never substitutedvalues.
So, what are we looking for, now that
we’ve finally started the engine? The
specific answer, of course, will depend
largely on the details of the customer
complaint and/or DTC(s) that are
stored. We might, for example, be focus-
ing on fuel trim numbers (and trends) if
our code suggests an underlying air/fuel
metering problem. We might be looking
most closely at engine coolant tempera-
ture, and time-until-warm measure-
ments when that seems warranted. Per-
haps our problem lies in the evap area,
or involves EGR flow. But ultimately, it
doesn’t matter what the specific issue is;
we’ll have to focus in on the systemic in-
teractionsthat determine the overall
characteristics of a particular data set.
Here’s a concrete example to illus-
trate what I mean. The vehicle in ques-
tion is a 1999 Chevy Venture minivan
with the 3.4L V6. There was a DTC
P0171 (Exhaust Too Lean, Bank 1) in
memory with an active MIL. The sum
of Short Term and Long Term Fuel
Trims in freeze frame was in excess of
50%. Fuel pressure and volume had
been verified as within specification.
When evaluating a fuel trim trouble
code, one of the first steps must always
be to verify that the oxygen sensor (on
which the DTC is based) is functioning
correctly. During the test drive, I ob-
served the O
2sensor switching rich, but
not as often as would be expected if the
very large fuel trim corrections shown
were actually effective. Indeed, on the
face of it, datastream seemed to confirm
the DTC. Longtime readers, however,
can probably anticipate what my next
tests were: I checked the actual lambda
value of the exhaust gases. Then I
looked for a dynamic response as I artifi-
cially enriched the system with a blast of
propane, then enleaned it by discon-
necting a major vacuum hose. (See
“What Goes In…Harnessing Lambda as
a Diagnostic Tool” in the September
2005 issue of M
OTOR. Search the index
at www.motormagazine.comfor all M
O-
TORmagazine articles mentioned.) Hav-
38 August 2008
DATASTREAM IN-DEPTH ANALYSIS
Data collection and analysis might yield some
helpful information, if you can find the wheat
within the chaff. This is only a small portion of a
larger data set with 100 values per PID.
When evaluating a
fuel trim trouble
code, one of the
first steps must
always be to verify
that the oxygen
sensor (on which the
DTC is based) is
functioning correctly.
Chart & screen capture: Sam Bell

17. 39August 2008
ing found the idle lambda at a ridicu-
lously low value of .85 (indicating a mix-
ture with 15% more fuel than needed), I
was not surprised to see that the O
2sen-
sor didn’t register a rich condition until
the engine was very nearly flooded with
propane. When I removed the purge
hose, engine rpm climbed and the en-
gine smoothed out, while lambda
marched toward the stoichiometric ideal
value of 1.00. Once the faulty O
2sensor
was replaced, all aspects of driveability
improved, and the minivan returned to
its previous fuel consumption levels.
Dynamic tests verify DTC accuracy.
In some instances, we may be able to
utilize bidirectional controls embedded
within our scan tool packages to actuate
various components. In other cases, we
may need to improvise, using signal
simulators, power probes, jumpers,
propane or just good, old-fashioned test
driving as required to initiate change
within the system we’re working on.
(I’m not saying that it will always be as
easy as it was with the Venture. You and
I know there will be problems that don’t
set DTCs, problems that do set DTCs
that have no apparent connection to the
actual root fault and, of course, prob-
lems that set appropriate codes yet are
still really hard to diagnose.)
Floodlights and Spotlights
One of the most powerful features of
most scan tools is, as nearly as I can
tell, one of the least used. This is the
so-called flight recorder, data logger or
movie mode. By whatever name it’s
known, this is an analytical tool of con-
siderable value.
Take a look at the portion of saved
scan data portrayed in the chart on page
38. As you see, any value in that infor-
mation is well hidden. This might be
termed a “floodlight” view, showing too
many values for too many parameters.
But look at the “spotlight” view above,
where I’ve selected and graphed a few
of the same PID values. This was a ve-
hicle where there was no DTC stored in
memory. By including both upstream
O
2sensors, I have provided myself a
cross-check, as there is less likelihood of
Graphical representations of scan data “movies” can speed analysis. As an added bonus,
using your scanner’s flight recorder mode allows you to concentrate on your driving. The
data set here clearly points to a lack of adequate fuel volume. This graphical representa-
tion is derived from the exact same movie capture seen in the chart on the previous page.
One of the most
powerful features
of most scan tools—
the so-called flight
recorder—seems to
be one of the least
used. But it’s an
analytical tool of
considerable value.

18. both being bad. Similarly, MAF and
rpm track nicely with one another, again
providing a good cross-check. The data
values at the cursor (the vertical line at
frame 2) are called out at the left side of
each PID’s plot. The upstream O
2sen-
sors are switching nicely at 2000 rpm (as
shown at frame 51), but the graphic
interface reveals an obvious problem at
higher speeds as the O
2sensors flat-line
lean. A new fuel pump restored the
missing performance.
Slow Motion and
High Speed
Moviemakers speed up or slow down
the action on the screen by shooting at
different numbers of frames per sec-
ond. When film shot at 20 frames per
second is played back at 60 frames per
second, the action seems to be occur-
ring at three times the speed. Just as a
56k dial-up modem is slower than a
DSL Internet connection, scan data
transfer rates also vary according to the
interface used. Generic communica-
tion modes often travel at a crawl, es-
40 August 2008
DATASTREAM IN-DEPTH ANALYSIS
M
ost MOTORreaders have at least a passing fa-
miliarity with the concept of OBD II monitor
completion status. Even so, a brief refresher may be
in order. OBD II monitors are simply formalized sets
of self-tests all related to a particular system or
component.
CCo on nt ti in nu uo ou us s m mo on ni it to or rs s. . With a few very rare ex-
ceptions (mostly for 1998 and earlier models), the
so-called
continuous monitorsalways show up as
“complete,” “done” or “ready.” Take this status re-
port with a grain of salt. Unplug the IAT sensor, start
the engine and check that the “Comprehensive
Component Monitor” readiness status shows com-
plete. Is the MIL on? Are there any pending codes?
How long would you have to let the vehicle idle be-
fore it will trip the MIL and show a P0113 (IAT Sen-
sor Circuit Voltage High) DTC?
As it turns out, depending on the specific make,
model and powertrain package, there are several
specific criteria that must be met
beforethe code
will set. In one instance, the PCM must detect a VSS
signal of 35 mph or more and an ECT value of 140°F
or more, the calculated IAT must be less than 38°F
and all of these conditions must be met for at least
180 seconds of continuous duration, during which
no other engine DTCs are set—all while MAF is less
than 12 grams per second. (This particular example,
incidentally, is a two-trip code. Some other manu-
facturers may make this and other DTCs under the
component monitor’s jurisdiction into one- or two-
trip codes, sometimes with even more complicated
entry criteria.)
Continuous monitors include the comprehensive
component monitor, the fuel monitor and the mis-
fire monitor. Each monitor runs continuously
when
conditions are appropriate
, but not during all actu-
al driving. For example, the misfire monitor is often
suspended during 4WD operation, since feedback
through the axles over rough roads might cause
uneven disruption of the CKP signals, which could
otherwise be misidentified as misfires. Similarly, ex-
tremely low fuel tank levels may suspend both mis-
fire and fuel system monitors to avoid setting a
DTC for running out of gas.
NNo on nc co on nt ti in nu uo ou us s m mo on ni it to or rs s. . As I pointed out last
month, it’s important to note the readiness status of
the other, noncontinuous monitors as well. These are
the monitors whose status will change to “incom-
plete,” “not ready” or “not done” when the codes
are cleared. If a vehicle arrives at your shop showing
one or more incomplete monitors, it’s likely that
someone has already cleared the codes
beforeit got
to you. (There are a few vehicles—for example, some
1996 Subarus—which may reset monitor status to in-
complete at every key-off, or other vehicles which
may have certain monitors which cannot be made to
run to completion in normal driving, such as the evap
monitor on some Toyota Paseos.) If a vehicle shows
up with incomplete monitors, however, you should
certainly document that fact on your work order and
be sure to advise the customer that there’s a very real
possibility that one or more other codes may recur af-
ter the current repair has been completed. For more
on this subject, see my article “How Not to Get MIL-
Stoned” in the April 2004 issue of M
OTOR.
More importantly, for our present purposes, the
existence of incomplete monitors means that you
may not be getting the whole picture as to what
ails the vehicle you’re looking at. Keep an open
mind, remembering that there may be other, as yet
unknown issues hidden behind that incomplete
monitor, and try not to rush your diagnosis. As
mentioned in last month’s installment, there
may
be some valuable data accessible via Mode 6 even
if the monitor is not complete, but there is a very
real possibility that Mode 6 data for any incom-
plete monitor may turn out to be unreliable. And,
of course, don’t overlook any pending DTCs. Re-
member, these do not illuminate the MIL, so you
must seek them out on your own.
Monitors 1.01

19. 42 August 2008
DATASTREAM IN-DEPTH ANALYSIS
pecially in comparison to CAN speeds.
If you’re stuck with a generic interface,
you can often accomplish more by
looking at less.
The key here is PID selection.
Choose the smallest number of PIDs
that will give you the information you
actually need. Three or four are usu-
ally sufficient. This is your version of
the filmmaker’s high-speed action
trick, as you get more updates per
unit time the fewer PIDs you select.
With several hundred possible PIDs
from which to choose, it’s just too
easy to miss an intermittent data
glitch, or to drown in a sea of too
much information (see “Live Data vs.
‘Live Data’” on page 44).
Most M
OTORreaders are familiar
with the ways in which some of the ma-
jor OEMs have organized data PIDs
for display in their enhanced scan tool
interfaces. Groupings such as Misfire,
Driveability, Emissions, Accessories
and the like are good examples of the
types of data sets you may want to con-
struct while analyzing different sorts of
problems. Tracking down a nasty inter-
mittent problem? Don’t hesitate to
pare down the OEM groupings even
further to speed data updates.
Code-Setting Criteria and
Operating Conditions
If we’re trying to resolve a MIL-on
complaint, it’s critical that we first re-
view both the exact code-setting crite-
ria and the operating conditions as re-
vealed in our previously recorded
freeze frame data. We’ll need to drive
in such a way as to complete a good
“trip” so the affected monitors can
run to completion. (For a more de-
tailed discussion of OBD II trips and
monitors, see “Monitors 1.01” on
page 40.) If we fail to meet the condi-
tions under which the self-test (moni-
tor) will run, we cannot hope to make
progress. Using the previously record-
ed freeze frame parameters gives us a
good general idea of the operating
conditions required. Merely duplicat-
ing speed, load, temperature and oth-
er basic characteristics may not be
enough. This is why we need to re-
view and understand the details of the
code-setting criteria and the monitor’s
self-test strategy. For example, some
monitors cannot run until others have
already reached completion. A typical
example would be a catalytic convert-
er monitor that is suspended until the
oxygen sensor monitors have run and
passed.
Some trouble codes, or even pending
codes, suspend multiple monitors. Oth-
er vehicle faults may then go undetect-
ed until all monitors can run again. A
P0500 (VSS Malfunction) in a Corolla,
for example, will effectively suspend
even the misfire monitor.
I
t seems like a no-brainer: When you’re done with all
your diagnostic tests and you’ve made the necessary
repairs, you should turn off the MIL, right? That’s
what your customer probably expects, and as we all
know, meeting customer expectations is an important
part of running a successful business.
But there are often times when you should leave
the MIL on. If your area uses an OBD II “plug & play”
emissions test, the regulations usually require that no
more than one monitor can be incomplete as of the
time of testing for model year 2001 and newer vehi-
cles, with no more than two incomplete monitors for
1996 to 2000 models. In some areas, retest eligibility
requires that the converter monitor must show “com-
plete” before a retest is valid.
If an emissions test or retest is looming in your cus-
tomer’s future, you and he must work out the pros
and cons of clearing the codes and resetting the mon-
itors to “incomplete.” If you clear the codes, the mon-
itors will reset as well. This will require that someone
will have to drive a sufficient number of monitors to
completion before a retest will be valid. If local
weather conditions, for example, will prevent the
monitors from running in a timely way, your customer
might be better off if you leave the MIL on. Then your
customer would have to drive only those portions of
the drive trace needed to run the monitor under
which the current DTC set.
For example, if you’re in the frigid climes of an up-
per Midwestern winter and a customer’s vehicle failed
an emissions test because of a faulty O
2sensor heater,
you’ll both be ahead if you don’t clear the code, letting
it expire naturally as the heater monitor runs success-
fully to completion on the next two trips. This will
avoid the necessity of rerunning all the rest of the
monitors. Of course, if the vehicle failed the evap mon-
itor, you’ll be better off clearing the code, because pro-
longed subfreezing temperatures may make running
that particular monitor successfully virtually impossible
for weeks at a time.
Lights Out?

20. The net result is that we may have to clear the current
DTCs and extinguish the MIL before our test drive can
bear fruit. (But again, please be sure to read and record
all the freeze frame data, the status of all monitors, the list
of both current and pending DTCs and any available
Mode 6 data beforeclearing the MIL (see “Lights Out?”
on page 42).
We’ll need to drive long enough to let the monitors in
question reach completion. In some cases, this may require
an extended period of time. Many Ford products, for exam-
ple, normally require a minimum of a six-hour cold-soak be-
fore the evap monitor can run, although there may be ways to
force this issue in some instances. Many Chrysler oxygen sen-
sor monitors run only after engine shut-down (with key off),
so that no amount of drivingwill ever bring them to comple-
tion. Certain monitors, and apparently even certain scan tools,
may require a key-off sequence before the monitor status will
update from incomplete to complete. M
OTORoffers an excel-
lent resource to help you understand these details—the OBD
II Drive CycleCD Version 7.0, available from your local
M
OTORDistributor (1-800-4A-MOTOR).
In some cases, local weather conditions may make monitor
completion seem impossible until a later date, usually be-
cause of ambient temperature requirements, although some-
times as a result of road conditions. In most cases, however, it
will still be possible to complete the monitor by running the
vehicle on a lift or dynamometer. This option may occasional-
ly result in setting, say, an ABS code, but most monitors can
be run to completion swiftly and successfully on a lift. This
option may also offer a safer, faster alternative to actual driv-
ing, as trees and telephone poles are less likely to jump in
front of a vehicle on a stationary lift.
44 August 2008
DATASTREAM ANALYSIS
Circle #22
I
ntermittent interruptions of sensor data
can cause tricky driveability problems.
Some glitches may set a DTC while others
may not. While viewing datastream
may
reveal an intermittent sensor problem, it
should not be relied upon to do so. The is-
sue, once again, is in the data rate. Even a
moderately fast interface, say the 41.6
kbps (kilobytes per second) J-1850 PWM
used on many Ford products, can easily
miss a several-millisecond dropout
if it’s
not that particular PID’s turn in the data-
stream
. Where symptoms or DTCs point to-
ward an intermittent sensor glitch, you’re
probably better off breaking out your
scope or graphing multimeter.
Live Data vs. ‘Live Data’

21. Conclusions
Proper in-depth datastream analysis
can often light the way toward correct
diagnosis of driveability concerns.
Recording all available DTCs, pending
DTCs, freeze frame data and Mode 5
and Mode 6 results beforeclearing any
DTCs is essential. Specific setting cri-
teria for each DTC are manufacturer-
determined, regardless of whether the
code assigned is generic or manufac-
turer-specific. Freeze frame data sets
can be used to recreate the operating
conditions under which a previous fail-
ure occurred and can help illuminate
the conditions under which certain
self-tests are conducted. Mode 5 and
Mode 6 test results can help in analyz-
ing the type and extent of certain fail-
ures. KOEO datastream analysis can
sometimes reveal sensor faults or ration-
alityconcerns that might otherwise be
overlooked.
Looking at KOEO and KOER data-
stream on a regular basis makes known-
good values familiar. Once you know
the correct values, the conditions ac-
companying problems identified by
freeze frame are easier to spot. KOER
data can highlight current problems, es-
pecially when used in conjunction with
graphical scanner interfaces. Generic
data PIDs cannot include substituted
values, and so may point up faults easily
overlooked in more enhanced inter-
faces. Careful selection of custom-
grouped PIDs can provide faster scan-
ner update rates.
Pick your tools wisely. To verify hard
faults, monitor datastream as you run
actuator tests. Look for any mismatch
between the command sent to a com-
ponent and its actual response. For in-
termittent problems, record and graph
data. In tough cases, test circuits with
your scope or meter to verify actual
voltage for comparison to specs.
Used properly, these techniques
will help you arrive quickly and confi-
dently at an accurate diagnosis of the
root cause of most driveability com-
plaints.
45August 2008
Circle #23
When trying to
resolve a MIL-on
complaint, it’s
critical to first
review the exact
code-setting
criteria and the
operating
conditions as
revealed in the
freeze frame data.
This article can be found online at
www.motormagazine.com.

22. O
nce in a while we may
encounter a total fail-
ure of a MAF sensor,
one that is, perhaps,
short circuited or inter-
nally open. Much more
common, however, are failure modes in
which the MAF sensor has become un-
reliable, underreporting or overreport-
ing the true airflow into the engine. In-
deed, as we shall see, many MAF sen-
sor failures actually result in both un-
derreporting andoverreporting!
Before we get down to brass tacks, a
brief review of the basics of MAF sys-
tems is in order. Fuel control systems
for most modern gasoline engines are
centered either on MAF or MAP (man-
ifold absolute pressure). MAF systems,
which, as their name suggests, measure
the weight of incoming air and then
meter the appropriate amount of fuel to
ensure efficient combustion, are poten-
tially more precise, although MAP sys-
tems, which calculate fuel requirements
based on engine load, have historically
demonstrated greater reliability.
As you already know, combustion is
most efficient when the ratio of air to
fuel is approximately 14.7:1 by weight.
Mass and weight are essentially synony-
mous in the presence of a sufficiently
strong gravitational field such as the
Earth’s. Thus, knowing the weight of
the air entering the engine allows the
engine controller to meter the exact
amount of fuel required to achieve effi-
cient combustion. The controller com-
mands the fuel injectors to open for an
amount of time calculated to be suffi-
cient to allow the correct weight of fuel
to enter the engine, providing that the
fuel’s pressure is known. Fuel delivery is
fine-tuned by applying fuel trim correc-
tions derived from the closed-loop feed-
back of the oxygen sensor(s).
If the entire system is working as de-
signed, fuel trim corrections, expressed
as a percentage deviation from the base
fuel delivery programming, will be with-
in 10% (either positive or negative) of
the programmed quantity. In the ab-
sence of a MAF-specific diagnostic trou-
ble code (DTC), what would first lead
us to even suspect that a faulty MAF
sensor might underlie a particular drive-
ability problem?
To function correctly, allof the air
entering an engine’s combustion cham-
bers must be “seen” by the MAF sen-
sor. This means that any vacuum or air
leak downstream of the sensor will re-
sult in insufficient fuel metering, caus-
ing a lean condition in open-loop opera-
tion and higher-than-normal fuel trim
values in closed-loop. When we en-
counter a MAF sensor-equipped vehi-
cle exhibiting these symptoms, we need
to check for unmetered airflow first.
Remember, too, that unmetered airflow
may not require an external air leak. An
incorrectly applied or faulty PCV valve
can result in incorrect MAF data where
the PCV intake through the breather
hose is upstreamof the MAF.
So, the first two rules of MAF sensor
diagnosis are:
1. Find and eliminate all external air
28 July 2006
MAF
DIAGNOSIS
Photoillustration by Harold Perry; photos courtesy Wells Manufacturing Corp.
SUCCESSFUL
MAF
SENSOR
DIAGNOSIS
BYSAMBELL
A broad range of seemingly unrelated or
contradictory driveability complaints
may arise from MAF sensor
performance faults. Use this guide to
navigate out of a diagnostic thicket or,
better still, to avoid one entirely.

23. or vacuum leaks downstream of the
MAF sensor. When in doubt, use a
smoke machine, or lightlypressurize
the intake manifold and spray with a
soap & water solution.
2. Verify that the manufacturer-speci-
fied PCV valve is correctly installed and
functioning as designed. (This is one in-
stance where precautionary replace-
ment may be cost-justified.)
Only after these two steps have been
completed can you safely proceed with
other diagnostics. The foremost clue
that the fault lies with the MAF sensor
itself will be excessive fuel trim correc-
tions, usually negative at idle, more or
less normal in midrange operation and
positive under high load conditions (see
“How Contamination Affects Hot-Wire
& Hot-Film MAF Sensors” on page 32).
While there are several distinct MAF
sensor technologies ranging from hot-
wire or hot-film to Karman vortex and
Corialis sensors, and while MAF sensor
outputs may take the form of variable
frequency, variable current or a simple
analog voltage, the diagnostic principles
remain largely the same.
Let’s start with Ford vehicles, for a
couple of reasons. First, they are so
widespread that most of us are familiar
with them. Second, most MAF sensor-
equipped Ford products make use of a
PID (Parameter IDentification) called
BARO (barometric pressure). Up to
2001 models, this was an inferred, or
calculated, value generated by the PCM
(powertrain control module) in re-
sponse to the maximum MAF flow
rates observed on hard wide-open
throttle (WOT) acceleration. Where
this calculated BARO PID is available,
it is of great diagnostic value, since it
can confirm MAF sensor accuracy, if
only under high flow rate conditions.
To use the BARO PID, you must
first know your approximate local baro-
metric pressure. You might consult the
BARO PID on a known-good MAP
sensor-equipped vehicle. Alternatively,
your local airport can provide this data.
Do not rely on local weather stations,
however, since these usually report a
“corrected” barometric pressure. If
weather information is the only avail-
able source, a rule of thumb is to sub-
tract about 1 in. of mercury (1 in./Hg)
for every 1000 ft. of elevation above sea
level. This will yield a rough estimate of
your actual local barometric pressure.
For greater accuracy, you can purchase
a functional barometer for something
less than $40. Compare this data with
the BARO PID. A large discrepancy
here—say, more than 2 in./Hg—should
direct your suspicions toward the MAF.
Confirm your hypothesis as follows:
First, make sure you have followed the
steps outlined in the two rules above.
Next, record all freeze frame data and
all DTCs, including pending DTCs. If
the OBD monitor readiness status for
oxygen sensors shows READY, proceed
to the next step. If it doesn’t, refer to
the procedures in the following para-
graph now. Next, perform a KAM
(Keep Alive Memory) reset and drive
the vehicle. Make sure your test drive
29July 2006

24. includes at least three sustained WOT
accelerations. (It’s not necessary to
speed to accomplish a sustained WOT
acceleration. Rather than a WOT snap
from idle, an uphill downshift at 20 to
30 mph is usually sufficient. The WOT
prescription can be met at throttle
openings as low as 50% to 70%.) The
BARO PID should update from its de-
fault reading by the end of the third
WOT acceleration. If it’s now close to
your local barometric pressure, the
MAF sensor is not likely to be faulty. If
BARO is not close, try one of the clean-
ing techniques explained in the sidebar
“Keeping It Clean” on page 34, then
again reset KAM and take a test drive. If
the BARO is still out of range, a replace-
ment MAF sensor is in your customer’s
future. Unfortunately, in many 2002 and
later Fords, the calculated BARO PID is
supplanted by a direct BARO reading
taken from a sensor incorporated into
the ESM (EGR System Management)
valve, greatly lessening its diagnostic val-
ue for our current purposes.
If the oxygen sensor monitor status
showed INCOMPLETE above, you’ll
have to verify O
2sensor accuracy and
performance before performing the
KAM reset procedure. Use a 4- or 5-gas
analyzer to determine whether the
air/fuel ratio is correct in closed-loop
operation. The notes about lambda ()
below should help.
Outside of the Ford family, MAF
sensor diagnosis is more difficult. Large
fuel trim corrections—either positive or
negative—are often the only initial
pointer to MAF sensor problems.
Again, any and all air leaks downstream
of the MAF sensor must be repaired
first. Since accurate fuel trim correc-
tions depend on correct O
2sensor out-
puts, you must verify the functionality
of these sensors first. The easiest and
fastest way to do this is by checking
lambda, a type of measure of the air/fuel
ratio. (For a detailed explanation, see
my article in the September 2005 issue
of M
OTOR.) If the O2sensors are func-
tioning correctly, lambda at idle should
be very nearly equal to 1.00 in closed-
loop. You may wish to check this also at
1500 to 1800 rpm to verify adequate
mixture control off idle. Once lambda is
found to be correct, the O
2sensors are
proven good. Then any fuel trim adjust-
ments must result from unmetered or
incorrectly metered airflow or from in-
correct fuel delivery.
Distinguishing between fuel delivery
problems and MAF sensor problems
can be very frustrating. Start by verify-
ing fuel pressure andvolume. (Those
who rely on pressure alone may regret
30 July 2006
SUCCESSFUL MAF SENSOR DIAGNOSIS
Fig. 1 Fig. 2
Fig. 3 Fig. 4
Screen captures: Sam Bell

25. it.) Use your scan tool to record critical
data PIDs and graph them for analysis.
Here are a couple of examples:
In Fig. 1 on page 30, taken during a
period of closed-loop operation, short-
term fuel trims (blue and green traces)
for each bank were above 13% at 1100
rpm (red trace), yet dropped sharply
negative at 3600 rpm, proving that inade-
quate fuel delivery was notthe problem.
The values indicated in the legend box-
es correspond to the readings obtained
at the indicated cursor position (vertical
black line). The vertical white line indi-
cates the trigger point for the recording.
Subsequent diagnostics focused on the
MAF sensor and the PCV system.
Take a look at the scan data graph
shown in Fig. 2. It shows a car whose
faulty fuel pump was unable to deliver
sufficient fuel under high load condi-
tions. Notice the very low O
2sensor
readings (displayed in blue) corre-
sponding to the cursor (black vertical
line just to the right of the zero time
stamp). Fuel pressure was within spec
at idle and at about 2000 rpm, but vol-
ume was very low. The sudden drop-
off in O
2activity in response to hard
acceleration is a characteristic ob-
served in many instances of MAF sen-
sor faults as well.
Ultimately, known-good snapshots,
waveforms and other data sets are in-
valuable. Take a look at the scan snap-
shot in Fig 3. Does it show good fuel
trim and appropriate MAF sensor
readings?
Since total fuel trim stays well within
the 0 ±10% range throughout the
trace, it’s a good bet that the MAF sen-
sor is working well, at least under the
sampled conditions.
How about the data set shown in
Fig. 4? In fact, the snapshot was taken
during open-loop, closed-throttle de-
celeration when fuel was not being in-
jected, so the O
2sensor PID makes
sense. It’s actually a substituted default
value inserted whenever the vehicle is
in closed-throttle decel mode. What
about the reported MAP value? A
reading of 4.00 in./Hg shows very high
engine vacuum, which jibes with the
reported TPS PID. The fuel trim data
is within the usually accepted range of
0 ±10%. Good data can come in a vari-
ety of formats.
Of course, waveform captures from
your scope are often all that are needed
to confirm a faulty MAF sensor. In our
shop, we’ve found that a snap-throttle
MAF test for Ford products should al-
ways produce a peak voltage of at least
3.8 volts DC. The snap-throttle test is
32 July 2006
SUCCESSFUL MAF SENSOR DIAGNOSIS
Fig. 5 Fig. 6
H
ot-wire and hot-film MAF sen-
sors calculate airflow based on
monitoring the current re-
quired to maintain a constant tem-
perature in the sensing element.
When dirt accumulates, the addi-
tional surface area allows greater
heat dissipation at low airflow rates.
The dirt, however, also functions as
an insulator, with an overall net re-
sistance to heat transfer at very high
airflow rates.
At idle and under relatively low
flow/load rate conditions where the
majority of operation may take
place, the surface area effect usual-
ly predominates, causing a rich con-
dition with fuel trim corrections
usually in the range of 10% to
5%. At sustained high flow/load
rates, the insulative effect usually
takes over, causing a lean mixture
needing fuel trim corrections as
high as +30%.
Worse still is a complex case of
“mass confusion” that may arise un-
der hard acceleration when long-
term negativefuel trim corrections,
learned in closed-loop under low-
flow-rate conditions, are applied
precisely when positivefuel trim cor-
rections would be more appropriate.
So, for example, when the system
goes to open-loop during hard accel-
eration where the MAF is already
underreportingairflow by up to
30%, the PCM may subtract an addi-
tional10% to 15% (LTFT) from the
normal fuel delivery calculation,
leaving the system as much as 45%
leaner than desired!
In midrange operation, the two
effects (surface area and insulative
properties) may roughly cancel each
out, with fuel trims being more or
less normal. Additionally, the exact
chemistry and configuration of dirt
buildups can vary, changing the bal-
ance of power between the surface
area and insulative effects.
How Contamination Affects Hot-Wire &
Hot-Film MAF Sensors

26. performed the same way as
for ignition analysis. The idea
is not to race the engine, but
simply to open the throttle
abruptly to allow a momen-
tary surge of maximum air-
flow as the intake manifold
gets suddenly filled with air.
It’s critical that the throttle be
opened (and closed) as quick-
ly as possible during this test.
The waveform in Fig. 5 on
page 32 is from a known-good
MAF sensor. Note the peak
voltage of 3.8 volts. The rapid
rise and fall after the throttle
was first opened is normal
and reflects the initial gulp of
air hitting the intake manifold
walls and suddenly reaching maximum
density, greatly reducing subsequent
flow. The exact shape of the waveform
may vary from model to model, based
on intake manifold and air duct
(snorkel) design.
What’s the relationship between
MAF and engine speed? As Fig. 6
shows, rpm and airflow rate track one
another closely under the moderate ac-
celeration conditions during which this
screen capture was taken. The similarity
of the shapes of the two traces shown
here suggests, but does notprove, that
the MAF sensor is functioning well un-
der these conditions. If the airflow re-
port was consistently increased or de-
creased by the same factor, say 10% or
even 50%, the shape of its graph would
remain the same.
Consider the additional plots pre-
sented in Fig. 7 above. Does the extra
data shed any light on the MAF sensor’s
accuracy? Or is this just an example of
too much information?
Since short-term and long-term fuel
trims remain within single
digits throughout, we can be
reasonably sure that the MAF
sensor is functioning correctly.
Do we really benefit from
looking at the O
2sensor data
here? We could probably do
almost as well without it, since
we have both STFT and
LTFT, but the O
2trace (blue)
serves as an additional cross-
check on the validity of the
fuel trim calculations. More
importantly, the O
2sensor
trace proves both that an ap-
propriately rich mixture was
obtained on hard acceleration
and that applied fuel trim
corrections were effective
throughout the captured data set.
I said at the outset that hard failures
were relatively rare, but they do occur
from time to time, and I owe it to you to
discuss this type of failure as well as in-
termittent failures. Open-circuited or
short-circuited MAF sensors usually set
a trouble code, most frequently P0102
or P0103 (low input and high input, re-
spectively). P0100 is a nonspecific MAF
sensor circuit fault, while P0104 indi-
cates an intermittent circuit failure.
Checking scan data is a vital first step
toward successful diagnosis of any of
these codes. On pre-OBD II vehicles
especially, unplugging a faulty MAF
sensor will often restore a minimum de-
gree of driveability as the PCM reverts
to TPS, rpm and/or MAP as fuel deter-
minants. Certain mid-’80s GM vehicles
were notorious for intermittent MAF
sensor failures. These usually could be
easily recreated by lightly tapping with a
small screwdriver on the MAF sensor
housing at idle. A noticeable stumble
occurring with each tap clinches the
condemnation (Fig. 8, page 36).
Of course, backprobing the MAF
sensor connector for voltage drops at
both the power and ground terminals
KOER is a required step before any fi-
nal condemnation. The coincidence of
VBATT and MAF both showing 0.0
volts cannot be ignored. Neither should
the mouse nest in the MAF, nor the
gnawed wires throughout the engine
compartment.
Why is this a hard diagnosis? Conta-
34 July 2006
SUCCESSFUL MAF SENSOR DIAGNOSIS
Fig. 7
M
ost MAF sensor failures re-
sult from contamination.
Sometimes the dirt is visible,
but more often it’s not. Technicians
have tried a variety of cleaners, with
mixed success. Many use an aerosol
brake/electrical parts cleaner, wait-
ing until the MAF sensor is cold. A
Ford trainer in my area swears by
the most popular consumer glass
cleaner. Several top technicians re-
port good results from steam clean-
ing, while others prefer a spray in-
duction cleaner.
The vast majority of technicians
warn that the MAF sensor may be
damaged by any type of cleaning
where the electrical connector is not
held upright. This is particularly true
where strong chemicals are used, as
they may pool and work their way
into the delicate electronic circuitry.
To avoid future contamination, be
wary of oiled air filters or any that
appear likely to shed lint. Poor seal-
ing of air filter housings may con-
tribute to contamination. Never
spray an ill-fitting air filter with a sili-
cone lubricant or sealer; such sprays
are likely to render the MAF sensor
inaccurate. If an engine produces ex-
cessive blowby gases, these may con-
taminate the MAF sensor, as well. Be
sure any specified filter breather ele-
ment is installed. If none is specified,
but oil accumulates in the air intake
housing, the MAF sensor or associat-
ed intake ducts, be sure to investi-
gate and remedy the cause to pre-
vent repeat failures. Be sure to check
manufacturers’ TSBs, the iATN
archives and other sources as well.
Keeping It Clean

27. minated MAF sensors often
overreportairflow at idle (re-
sulting in a rich condition and
negative fuel trim corrections)
while underreportingairflow
under load (resulting in a lean
condition and positive fuel
trim corrections).
This double whammy makes
diagnosis more difficult for a
number of reasons: First, many
technicians incorrectly elimi-
nate the MAF sensor as a po-
tential culprit because they ex-
pect it to show the same bias
(either over- or underreport-
ing) throughout its operating range. Sec-
ond, a lack of a direct MAF fault DTC
(such as P0100) is often mistaken to
mean that the MAF sensor mustbe
good. Third, the symptoms mimic
(among other possibilities) those of a ve-
hicle suffering from low fuel pump out-
put coupled with slightly leaking injec-
tors or an overly active canister purge
system. Even sluggish, contaminated or
biased oxygen sensors may cause similar
symptoms. Without appropriate testing,
it’s hard to distinguish—just by driv-
ing—among certain ignition or knock
sensor faults and MAF sensor malfunc-
tions. Additionally, since MAF sensors
are somewhat pricey, many technicians
are afraid to condemn them, fearing ei-
ther the customer’s or the boss’ wrath if
their diagnosis is not borne out. Perhaps
the biggest obstacle is lack of a
comprehensive database of
known-good waveforms, volt-
ages and scan data against
which to compare the suspect.
My own data set features
known-good scan data and
scope captures made KOEO,
at idle and on snap-throttle. In
general, these three data points
should be sufficient to identify
a faulty MAF sensor even be-
fore it sets a fuel trim code.
A bad Bosch hot-wire MAF
sensor may be the result of a
failed burn-off circuit. Don’t
simply replace the sensor; make sure
the burn-off is functional. (The purpose
of the burn-off is to clean the hot-wire
of contaminants after each trip.) Burn-
off is usually a key OFF function after
engine operation exceeding 2000 rpm.
Burn-off circuit faults may be in the
PCM or a relay. The hot-wire should
glow visibly red during burn-off.
So what can we conclude from all
this? A broad and seemingly unrelated
or even contradictory range of fuel sys-
tem-related driveability complaints may
arise from MAF sensor performance
faults. Fuel trim data showing excessive
corrections from base programming
casts strong suspicion on MAF sensor
performance issues. After recording all
DTCs and freeze frame data, many ex-
perienced techs recommend unplug-
ging a suspect MAF sensor to see if ba-
sic driveability is improved. Scope
traces at idle and on snap-throttle accel-
eration help verify MAF sensor guilt or
innocence.
As usual, a library of known-good
scan data and waveforms is invaluable.
The Min/Max voltage feature on your
DMM may not be fast enough to catch
actual peak voltage on a snap-throttle
test, but is usually sufficient for verify-
ing performance of frequency-generat-
ing (digital) MAF sensors. If your scope
is capable of pulse-width triggering, us-
ing that function will provide exact cap-
tures of digital MAF sensors in snap-
throttle testing.
36 July 2006
SUCCESSFUL MAF SENSOR DIAGNOSIS
Fig. 8
Visit www.motor.comto download
a free copy of this article.
Circle # 27

28. T
he Greek root gen- un-
der lies many words in
common parlance—ge -
nericis one of them.
Your medical insurance
provider and your phar-
macist both know that when it comes to
prescriptions, generic equivalents can
save us all big money, with identical re-
sults. In the last few years, ge nerichas,
for complex reasons, become a pejora-
tive term, often used to convey the idea
of something of lesser quality than a so-
called name-brand alternative.
Yet, for diagnostic purposes, the
generic datastream sometimes offers a
better window into powertrain manage-
ment operating conditions than even
the name-brand “enhanced” or “manu-
facturer-specific” interface can. In fact,
even though I own several much more
powerful and expensive scan tools, I
routinely use the generic interface re-
siding on the cheapest of the bunch as
my go-to choice for initial code retrieval
and data analysis.
This particular machine, an aging
Auto Xray EZ6000, offers no bidirec-
tional controls above code clearing, but
has the signal virtues of speed and a
very high overall connectivity rate. It al-
so quickly compiles a printable report
which includes current operating PIDs,
DTCs (including pending codes) and
freeze frame data, all of which are obvi-
ously useful. Individual monitor com-
pletion status requires a separate query,
as do both Mode $05 (oxygen sensor
test results) and Mode $06 (monitor
self-test results) data. Its nice graphing
program makes data analysis easy after
a road test, and I’m actually happy that
you cannot both read and record data
simultaneously, as the trees in my
neighborhood view that particular be-
havioral combination as an excuse to
jump out in front of you.
One of the primary benefits of the
20 July 2014
BYSAMBELL
A resourceful diagnostician knows
complicated and expensive equipment
isn’t always needed. Tools with a narrower
focus, combined with an enlightened
approach, allow him to get the job done.
B
YSAMBELL
A resourceful diagnostician knows
complicated and expensive equipment
isn’t always needed. Tools with a narrower
focus, combined with an enlightened
approach, allow him to get the job done.
DOING IT ALL WITHDOING IT ALL WITH
Photoillustration: Harold A. Perry; images: Thinkstock, David Kimble, Sun & General Motors

29. generic datastream stems from the re-
quirement that it not display substituted
values. While many so-called enhanced
interfaces offer access to a greater num-
ber of data PIDs, some of these may re-
flect substituted values not based on
current operating data. For example,
most Chrysler products will substitute a
reasonable guess for the actual intake
manifold vacuum value when the MAP
sensor is unplugged. If you look in the
enhanced datastream, you’ll see that
value varying quite believably as you rev
the engine or drive the vehicle. If you
look at MAP_Volts, however, you’ll see
a fixed value reflecting reference volt-
age (Vref) for the sensor circuit. But
how often do you actually look at that
PID instead of the vacuum reading?
While substituted values are prohibit-
ed in the generic datastream, calculated
valuesare not. Thus, for example, an
ECT PID of 40°F reflects the calcu-
lated temperature of an open ECT sen-
sor circuit. In such cases, Toyota, for ex-
ample, has for many years, then substi-
tuted a value of 176°F in its enhanced
datastream, but not in the generic data.
In our unplugged Chrysler MAP sensor
example above, using a generic inter-
face, you’d see an unmoving value of
something in the neighborhood of
255kPa or higher, corresponding to a
boost pressure of about 25 psi above at-
mospheric.
As a technical consultant to our state
EPA, several times a year I encounter
vehicles which have failed our OBD II
plug & play state emissions test for a
MIL-on condition with one or more cur-
rent DTCs that simply do not appear in
the “enhanced” interface, but which are
readily retrieved using a generic hook-
up. I’m afraid I can’t shed a lot of light
on why this would occur since, clearly, it
should not. Thus far, I have not encoun-
tered this issue in any 2008 or later vehi-
cles. There seem to be a few makes
21July 2014
GENERIC DATASTREAMGENERIC DATASTREAM

30. which are more prone to this problem,
but my data set is too sparse to be cer-
tain of any meaningful correlation. For
the moment, suffice it to say that the
state’s testing interface is also a generic
one, and, apparently, there are instances
in which a DTC may set but not be re-
trieved via even the factory scan tool. On
these occasions, only a generic interface
will work. As the saying has it, truth is
stranger than fiction.
An additional advantage of using the
generic datastream becomes apparent
when you’re working on a vehicle for
which your scan tool doesn’t provide an
enhanced interface. Don’t laugh; I’ve
had students call me up to ask what to
do because they didn’t have a scan tool
that offered, say, a Saab or Daihatsu op-
tion. A gentle reminder that they could
at least start in the generic interface
usually nets an embarrassed oops! Be-
cause the generic interface contains the
data most critical to engine operations
(see the starred items in the “Generic
PIDs” list on page 24), it’s normally suf-
ficient to rule in or rule out a particular
area of concern such as fuel delivery, for
example, early in the diagnostic process.
While you might well prefer to work
with a dealer-equivalent scan tool in al-
most all cases, in the real world you may
not be able to justify buying a tool with
limited utility vis-à-vis your regular cus-
tomer base.
Let’s take a look at what the generic
interface typically offers these days (see
the screen captures on this page). The
J1979 SAE standard specifically defines
128 generic data PIDs, but not all man-
ufacturers use or support all of them.
Some, such as Mode $01, PID$6F (cur-
rent turbocharger compressor inlet
pressure), are highly specialized and
won’t apply to most current-production
vehicles, while others, such as PID $06
(engine RPM), are pretty universal. A
typical PID list of current values (Mode
$01) or of freeze frame values (Mode
$02) would include some or all of the 74
items listed in the generic PIDs list.
Your scan tool may use slightly different
acronyms or abbreviations to identify
various data items.
Most of the PIDs in the list are prob-
ably familiar to you, but a few may have
you scratching your head. As you see,
starting with a model year 2005 phase-
in, several new parameters have been
added to the original generic data list.
These include both commanded and ac-
tual fuel-rail pressure, EGR command
and EGR error calculation, commanded
purge percentage, commanded equiva-
lence ratio and a host of others, includ-
ing many diesel-specific PIDs.
In-use counters may also indicate
how many times each of the various on-
board monitors has run to completion
since the codes were last cleared. The
list on page 24 includes most of the
generic PIDs currently in widespread
use. However, since not all manufactur-
ers support all PIDs, and since their
choices may vary by model, engine
and/or equipment, the list given here
represents only a portion of the PIDs
potentially supported. Additionally,
manufacturers are free to establish and
define supplemental modes and PIDs
which may or may not be accessible via
a generic interface. All ECUs with au-
thority or control over emissions-related
issues, however, must be accessible via
the generic interface.
From our generic PIDs list, I want to
focus on commanded equivalence ratios
first. In essence, this is the PCM’s way
of reporting how rich or lean a mixture
it’s commanding. The PID is presented
in a lambda format, with 1.0 indicating a
stoichiometric (ideal) air/fuel ratio.
Larger numbers indicate more air—a
command to run at a leaner air/fuel ra-
tio—while numbers less than 1.0 indi-
cate a correspondingly richer mixture.
If you have a gas analyzer capable of
DOING IT ALL WITH GENERIC DATASTREAM
These two screen shots show the 60 lines of generic data available from a
known-good 2014 Mazda CX-5, as captured via a Snap-on SOLUS Ultra scan tool.
22 July 2014
Screen captures: Sam Bell

31. displaying lambda, it should coincide
extremely well with the Commanded
Equivalence PID.
As with all fuel trim-related issues, it’s
best to check this PID at idle, at about
1200 rpm and at about 2500 rpm. If
your actual tailpipe measurements don’t
coincide with the PID, be sure to check
for any exhaust leaks first. If there are
none, you’ll have to check for factors
that could account for the discrepancy,
such as fuel pressure faults, vacuum
DOING IT ALL WITH GENERIC DATASTREAM
24 July 2014
T
he five starred (★) critical
PIDs in the list below are
the most influential inputs. Vir-
tually all the others function
merely to fine-tune (trim) the
basic spark and fuel (base
map) commands mapped out
in response to these PIDs. The
cause of any fuel trim correc-
tions (STFT, LTFT) beyond the
range of approximately 5%
must be investigated.
Standards Compliance- such
as OBD II (Federal), OBD II
(CARB), EOBD (Europe), etc.
MIL- malfunction indicator
lamp status (off/on)
MON_STAT- monitor comple-
tion status since codes cleared
DTC_CNT- number of con-
firmed emissions-related DTCs
available for display
★RPM - revolutions per min -
ute: also, engine crankshaft (or
eccentric shaft) speed, sourced
from the CKP
★IAT- intake air temperature
★ECT- engine coolant tem-
perature
★MAPand/or ★MAF- mani-
fold absolute pressure or mass
airflow, respectively
★TPSor ★TP- throttle position
sensor, usually given as calcu-
lated percentage; see absolute
TPS below
CALC_LOAD- calculated, based
on current airflow, as percent-
age of peak airflow at sea lev-
el at current rpm, with correc-
tion for current BARO
LOOP- status: closed, closed
with fault, open due to insuffi-
cient temperature, open due
to high load or decel fuel cut,
open due to system fault
STFT_x(per bank) - short-
term fuel trim; the percent-
age of fuel added to or sub-
tracted from the base fuel
schedule (for speed, load,
temperature, etc.) in order to
achieve stoichiometry as de-
termined by the relevant
air/fuel or O
2sensor
LTFT_x(per bank) - long-term
fuel trim
VSS- vehicle speed sensor
HO2SBxSy- heated oxygen
sensor, Bank x, Sensor y, such
as B1S2 for a bank 1 down-
stream sensor
IGN_ADV- ignition timing,
measured in crankshaft de-
grees
SAS orSEC_AIR- commanded
secondary air status off/on; may
include information such as at-
mosphere, upstream or down-
stream of converter, command-
ed on for diagnostic purposes
RUN_TIME- seconds since last
engine start; some manufac-
turers stop the count at 255
seconds
DISTANCE TRAVELED WITH
MIL ON– in miles or km
FRP- fuel rail pressure relative
to intake manifold pressure
FRP_G- fuel rail pressure,
gauge reading
O2Sx_WR_lambda(x)- wide
range air/fuel sensor, bank x,
equivalence ratio (0-1.999) or
voltage (0-7.999)
EGR- commanded EGR per-
centage
EGR_ERR- deviation of sensed
or calculated position from
commanded position, percent
PURGE- commanded percent-
age
FUEL_LVL- fuel level input per-
centage; can provide especially
invaluable information in freeze
frame diagnostics of misfire
codes set under “ran-out-of-
gas” conditions; unfortunately,
not universally implemented
WARMUPS- number of warm-
ups since codes cleared; a
warm-up is an ECT increase of
at least 40°F in which the ECT
reaches at least 160°F
DIST SINCE CLR- distance
since codes cleared
EVAP_PRESS- evaporative sys-
tem pressure
BARO- absolute atmospheric
pressure (varies with altitude
and weather)
O2Sx_WR_lambda(x) - equiv-
alence ratio or current - wide
range air/fuel sensor , position x,
equivalence ratio (0-1.999) or
current (-128mA to +127.99mA)
CAT_TEMP BxSy- catalyst tem-
perature by bank and position
(may be wildly unreliable)
MON_STAT- monitor status,
current trip
CONT_MOD_V- control mod-
ule voltage; usually measured
on the B+ input for the Keep-
Alive-Memory (KAM) but may
be measured on a switched ig-
nition input line
ABS_LOAD- absolute load,
percentage, 0-25,700%
REL_TPS- relative throttle po-
sition percentage
AMB_AT orAMB_TEMP- am-
bient air temperature; where
used, usually measured in
front of the radiator, while IAT
or MAT (manifold air tempera-
ture) are usually collected in
the intake ductwork, or inside
the throttle body or intake
manifold, respectively
ABS_TPx- absolute throttle
position, percentage, sensor B
or C
APP_x- accelerator pedal posi-
tion sensors D-F
TP_CMD- commanded throt-
tle actuator percentage
MIL_TIM- time run with MIL
on, minutes
FUEL_TYP- fuel type
ETOH_PCT or ETH_PCT -
ethanol fuel %
ABS_EVAP- absolute evap
system vapor pressure, 0-
327.675kPa
EVAP_Por EVAP_PRESS- evap
system vapor pressure (gauge),
from -32,767 to +32,768Pa
STFTHO2BxS2– short-term
secondary (postcatalyst) oxy-
gen sensor trim by bank
LTFTHO2BxS2- long-term sec-
ondary oxygen sensor trim by
bank
HY_BATT_PCT- hybrid battery
pack remaining life, percent-
age
E_OIL_Tor ENG_OIL_TEMP-
engine oil temperature
INJ_TIM- fuel injection timing,
in crankshaft degrees from
★210° BTDC to 302° ATDC
FUEL_RAT- engine fuel rate in
volume per unit time—e.g.,
liters per hour, gallons per
minute, etc.
TRQ_DEM- driver’s demand
engine, percent torque
TRQ_PCT- actual engine, per-
cent torque
REF_TRQ- engine reference
torque in Nm (0 to 65,535)
TRQ_A-E- engine percent
torque data at A=idle; B, C, D,
E = defined points
AFC- commanded diesel in-
take airflow control and rela-
tive intake airflow position
EGR_TEMP- exhaust gas recir-
culation temperature
COMP_IN_PRESS - turbo -
charger compressor inlet pres-
sure
BOOST- boost pressure con-
trol
VGT– variable-geometry turbo
control
WAST_GAT- wastegate con-
trol
EXH_PRESS- exhaust pressure
TURB_RPM- turbocharger rpm
TURB_TEMP- turbocharger
temperature
CACT- charge air cooler tem-
perature
EGTx- exhaust gas tempera-
ture, by bank
DPF- diesel particulate filter
DPF_T- diesel particulate filter
temperature
NOX- NO
Xsensor
MAN_TEMP - manifold sur-
face temperature
NOX_RGNT - NO
Xreagent sys-
tem
PMS- particulate matter sensor
Generic PIDs

32. leaks or a biased oxygen or air/fuel sen-
sor. If you observe a close correlation
with lambda, you’ll be able to use this
PID with confidence in lieu of actual
lambda readings while conducting addi-
tional tests.
In general, you should expect this
PID to read very close to 1.00 at idle in
closed-loop operation with conventional
oxygen sensors in the upstream posi-
tions. (Wide-range air/fuel [WRAF] ra-
tio sensors may target alternate values
under various driving conditions, typi-
cally targeting a leaner mix under light-
throttle cruise, for example. Additional-
ly, vehicles using gasoline direct injec-
tion [GDI] may deviate from stoichiom-
etry even at idle or under light-throttle
cruise conditions.) Keep in mind that
the name says a lot: This PID reports
the command, not necessarily the effect
of the command.
Once in a blue moon you may find
that commanded equivalence ratio
seems to travel exactly opposite from
lambda, so that a Com_Eq_Rat of .95
corresponds to an actual lambda value of
1.05, for instance. After the one instance
in which I’ve encountered this, I eventu-
ally learned to think of the PID value as
a deviation from 1.00, then move exactly
that far in the opposite direction. (An
unfortunate computer crash led to the
loss of my notes from that vehicle, and I
can no longer remember even which
foreign nameplate make it was, much
less the year, model and engine. What I
do remember is that it sure threw me
for a loop! I also remember rechecking
this at the time with another scan tool
with the same result, so I suspect that it
was simply the result of a mistranslation
somewhere along the way, and not a tool
glitch per se.)
One more note on the commanded
equivalence ratios PID: You’ll find it in
use for diesels as well. Stoichiometric
conditions for gasoline engines result in
an air/fuel ratio of approximately 14.7:1.
The advent of oxygenated fuels has ac-
customed us to seeing lambda values
showing slightly lean, up to as high as
1.04 in some cases, with no apparent
fault. Since fuel blends vary both region-
ally and seasonally, normal values for
your area may differ. With diesels, the
ratio is closer to 14.5:1, with propane
running best at 15.7:1 and natural gas
working out to about 17.2:1. If you’re us-
ing your gas analyzer on a vehicle burn-
ing one of these fuels, you’ll have to re-
set your lambda calculations accordingly.
Most gas analyzers with a computer
plotting interface readily accommodate
multiple fuel types, usually from the
setup menu. In the case of flex-fuel
DOING IT ALL WITH GENERIC DATASTREAM
26 July 2014
Value Description
0 .........Not available
1 .........Gasoline
2 .........Methanol
3 .........Ethanol
4 .........Diesel
5...........LPG (liquid propane gas)
6...........CNG (compressed natural gas)
7 .........Propane
8 .........Electric
9..........Bifuel running gasoline
10..........Bifuel running methanol
11 ........Bifuel running ethanol
12 ........Bifuel running LPG
13 ........Bifuel running CNG
14.........Bifuel running propane
15 .........Bifuel running electricity
16.........Bifuel running electric
and combustion engine
17 ........Hybrid gasoline
18 ........Hybrid ethanol
19 ........Hybrid diesel
20 ........Hybrid electric
21.........Hybrid running electric
and combustion engine
22 ........Hybrid regenerative
23 ........Bifuel running diesel
Any other value is reserved by ISO/SAE.
There are currently no definitions for
flexible-fuel vehicles.
Fuel Type Table:
Mode $01, PID $51
OBD- on-board diagnostics.
OBD II- second-generation OBD, as
specified by SAE J1979.
EOBD- Euro-specification OBD;
slightly different from SAE-spec.
JOBD- Japanese-specification OBD;
slightly different from SAE-spec.
DTC- diagnostic trouble code; P-
codes refer to powertrain manage-
ment faults; U-codes flag communi-
cation network errors; B-codes relate
to faults in body system manage-
ment; C-codes are chassis system
based.
PDTC- Permanent DTC; one that
cannot be cleared directly via scan
tool command; such codes will self-
clear after the affected monitors
have successfully run to completion
with no further faults. PDTCs are
written into a section of nonvolatile
memory, so they persist even if the
battery is disconnected and all capac-
itors are discharged.
PID- parameter identification; a val-
ue found in current or freeze frame
data; may indicate a sensor reading,
calculated value or command status.
In a nongeneric (enhanced) interface,
may indicate a substituted value.
$-or -$- prefix or suffix indicating
that an alphanumeric string is hexa-
decimal (presented in base 16.) The
J1979 specifications which establish
the OBD II protocol are written using
hexadecimal notation throughout.
Datastream- a set of PID values, DTCs,
test results and/or PDTCs; the display
of such data on or via a scan tool.
Freeze frame- a set of PID values in-
dicating then-current data written
into the PCM’s memory when a DTC
sets, similar to an aircraft flight
recorder.
Note:Freeze frame data is
erased when codes are cleared; be
sure to read and record before clear-
ing DTCs.
CAN- controller area network; also,
communication via the same.
Monitor- one or more self-tests exe-
cuted by the OBD system to deter-
mine whether a specific subsystem is
functioning within normal limits.
Monitor status changes to incom-
plete or “not done” when DTCs are
cleared, and returns to complete or
“done” once all relevant self-tests
have been run. A monitor status
showing completion is not a guaran-
tee of a successful repair unless there
are no codes and no pending codes,
and unless the vehicle has been oper-
ated under conditions similar to
those under which a previous fault
had occurred (see freeze frame).
Glossary

33. cars, check the ETOH_PCT PID to
help your analyzer figure out the cor-
rect stoichiometric ratio. Once you’ve
made the proper selection, you can
work from lambda without bothering to
know or remember the exact stoichio-
metric ratio involved.
I was certainly glad to see the appear-
ance of purge data in the generic list, as
knowing the commanded purge status
can assist in diagnosing several types of
driveability faults above and beyond
evap leaks and malfunctions. Remem-
ber, however, that this PID reflects only
the current commanded state, not nec-
essarily what’s actually happening.
The PIDs for EGR Command and
EGR Error are likewise helpful. De-
pending on the interface you use, how-
ever, EGR_Error may be reported
“backwards,” with 100% indicating that
command and position are in complete
agreement and 0% indicating that one
shows wide-open while the other
shows shut. (I’ve seen this on numer-
ous Hondas, where a 99.5% “error” ac-
tually meant that the valve was closed
as commanded.) As usual, a few min-
utes checking known-good vehicles
can help avoid many wasted hours
hunting problems that aren’t really
there.
Other new PIDs inform us of the
mileage since the last time the codes
were cleared as well as the distance
driven since the MIL first illuminated
for any current codes. Both of these
pieces of information can be useful, es-
pecially if yours is not the first shop to
look at a particular problem. In the case
of intermittent faults, they can also help
give you a better idea of just how fre-
quently the issue does arise.
Beyond PIDS
Potentially both more helpful and more problematic are the new Permanent DTCs found in mode $0A. These can- not be cleared directly via a scan tool, but will be self-erased once the corre- sponding monitors have successfully run to completion. Attempts to circum- vent plug & play emissions tests by sim- ply clearing codes without fixing the underlying causes led to the develop- ment of these Permanent DTCs. While there are times when I would rather just “kill the MIL,” the PDTCs make me take the extra time to more fully ed- ucate my customers and to verify the efficacy of my repairs, often by resort- ing to Mode $06 data analysis.
The key thing to remember when
working with Mode $06 data is that it’s
entirely up to the OEM to define all
TID$, CID$, MID$, etc. These defini-
tions can vary by year, engine, model
and/or equipment even within the
same OEM division, so be sure to veri-
fy the accuracy of any information
you’re using to interpret this data be-
fore you get yourself in trouble. Also
remember that many manufacturers
DOING IT ALL WITH GENERIC DATASTREAM
28 July 2014
Circle #16

34. populate their Mode $06 datastream
with “placeholder” values after codes
are cleared and until affected monitors
have run to completion. This is a strong
argument for waiting as long as possible
before clearing codes.
Probably 90% of the MIL-on com-
plaints we see in my shop are resolved
using “just” a generic scanner, coupled,
of course, with a few decades of experi-
ence! Nevertheless, since a generic
scan interface can take you only so far,
there are certainly other times when
we break out one of our more sophisti-
cated scan tools with bidirectional func-
tionality, access to additional PIDs,
guided diagnostics, etc.
Especially in an older vehicle, the
generic communications data rate
(baud speed) may also seem slow by to-
day’s standards. After an initial scan, this
limitation can often be overcome by se-
lecting a relatively small number of
PIDs relevant to the problem at hand.
All vehicles since 2008 support CAN
communications even in the generic in-
terface. The effective data transfer rates
here are plenty quick enough for almost
any practical purpose.
Since OBD II generic standards do
not apply beyond P-codes (and some U-
codes), any full-service shop needs one
or more scanners to deal with B-, C-
and most U-code issues. Remember,
though, that many OEMs illuminate
TRAC, VSC and/or ABS lights in re-
sponse to any P-code. This is nearly uni-
versally true in the case of drive-by-wire
(electronic throttle body) applications,
but may be found in many other in-
stances as well. In all such cases, you
mustresolve the P-code issue first, be-
foreworrying about any of these side-
effects codes. If you have an appropri-
ate interface, once you’ve killed the
MIL, clear those extra codes as well, so
the next tech doesn’t find them still in
memory if and when a legitimate B- or
C-code ever does set.
The bottom line is that there are sev-
eral potentially important advantages to
using a generic scan interface for initial
code retrieval and data analysis, so don’t
be afraid to get your feet wet! Since the
generic datastream focuses on the most
important inputs and commands, where
the bulk of problems occur, and since
all PID values reflect their associated
sensor states without substitution,
you’re less likely to be capsized by a
flood of irrelevant data.
As always, checking known-good ve-
hicles will help keep you on an even
keel and familiarize you with what
“good” looks like. While you may occa-
sionally wind up switching over to an
enhanced interface, you’ll likely find
that routinely starting in generic using a
fast and inexpensive basic scanner re-
sults in much greater efficiency.
Whether your shop is large or small,
this practice also lets you avoid exces-
sive wear and tear on the more expen-
sive and advanced scanners and keeps
them free for those longer-term diag-
nostic challenges where their enhanced
features are actually needed.
29July 2014
This article can be found online at
www.motormagazine.com.
Circle #17

35. 10 February 2015
T
he  commanded  equivalence
(EQ)  ratio  parameter  (PID)  is
required  in  the  generic  data-
stream  on  all  passenger  vehi-
cles since 2008 (see the EQ to
air/fuel  ratio  [AFR]  matrix  on
the next page and the SAE definition in the
box on page 13). An EQ of 1 equals 14.7:1
AFR.  This  PID  should  reflect  the  com-
manded  air/fuel  ratio.  That  being  said,
there  are  no  Bank1  and  Bank2  EQ  ratio
PIDs,  and  can’t  the  EQ  or  AFR  be  differ-
ent  for  each  bank?  Are  the  two  averaged?
Yipes!  How  is  this  going  to  work  on  a
vehicle?
This test was performed on a 2010 Toyota
Tacoma  with  a  4.0L  engine.  It’s  important
to  note  that  this  may  not  be  representative
of other vehicles; this is one test. Also, this
test is not intended to be critical of any im-
plementation  of  the  EQ  PID.  I  think  the
problem is in the original definition of this
PID—there  is  no  differentiation  for  Bank1 and Bank2.
Now let’s get into the layout of the screen
capture below, from the 2010 Tacoma:
In  the  top  chart,  rpm  is  in  red  (the  scale
on  the  left-hand  side)  and  vehicle  speed  is
in green (the scale on the right-hand side).
In the second chart, air/fuel ratio sensors
Bank1 and Bank2 are in milliamps, and both
are scaled on the left-hand side.
In  the  third  chart,  the  air/fuel  ratio  sen-
sors Bank1 and Bank2 are in volts, and both
are scaled on the left-hand side.
In  the  fourth  chart.  postcatalyst  O
2Sen-
sors Bank1 and Bank2 are in volts, and both
are scaled on the left-hand side.
In the last chart, the commanded EQ ra-
tio is scaled on the left-hand side.
All  data  to  the  left  of  the  dotted  line  in
the  screen  capture  is  a  baseline  test  drive
ending with a long idle period prior to intro-
ducing a skew in B1S1 AFR sensor (the ver-
New PIDs provide additional information that can be included in
your diagnostic efforts. But before it can be used, you must under-
stand how it was obtained and what it’s intended to represent.
Driveability Corner
Mark
Warren
[email protected]
Screen capture & chart: Mark Warren
DRIVEABILITY CORNER FEB 2015v2_Layout 1  1/22/15  10:22 AM  Page 1

36. tical  dotted  line).  The  solid  green
line is the point of measurement for
the  reading  in  the  small  boxes  to
the right of the parameter name on
the chart. Finally, I’ve drawn a solid
fine  black  line  horizontally  in  the
EQ chart to show EQ equals 1.
Data Analysis
Remember  that  the  air/fuel  ratio
sensors  (charts  2  and  3)  read  high
when  lean  and  low  when  rich—the
opposite  of  the  O
2sensors  that  are
low when lean and high when rich.
The  high  (lean)  spikes  in  the  AFR
sensor  data  reflect  deceleration  fu-
el-cut enleanment. Note the Bank1
and  Bank2  AFR  sensors  following
each  other  closely  in  the  baseline
data.
I  put  in  the  AFR  milliamp  and
voltage data to demonstrate the tiny
amount  of  amperage  used  and  the
conversion to volts scaling. Note the
postcatalyst oxygen sensors also fol-
lowing each other reasonably close-
ly.  It’s  noteworthy  that  at  105,000
miles,  the  rear  O
2sensors  don’t  go
above  .8V  and  lay  flat  on  zero  for
some  period  of  time.  Perhaps  these
11February 2015
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Circle #17
Circle #18
Circle #19
Circle #20
continued on page 13
DRIVEABILITY CORNER FEB 2015v2_Layout 1  1/22/15  10:22 AM  Page 2

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for those who demand the best.
IT’S NOT FOR EVERYBODY.    
  IT’S FOR YOU.
 
    
 
    
 
    
 
    
 
    
 
    
 
    
 
    
 
    
 
    
 
    
 
    
our fellow pros think at See what y
 
    
WD40Specialist.comour fellow pros think at 
 
    
sensors are showing some age.
Note  that  the  EQ  PID  in  the
baseline data period is pretty active
when  the  truck  is  being  driven.
Look  at  the  EQ  relative  to  the  top
chart of rpm and mph to get an idea
of  the  changing  load.  Notice  that
the  fuel-cut  events  that  are  reflect-
ed  well  in  the  AFR,  and  O
2sensor
13February 2015
Driveability Corner
SAE Definition:
Commanded 
EQ Ratio
Fuel systems that utilize convention-
al  oxygen  sensors  shall  display  the
commanded  open-loop  equivalence
ratio while the fuel control system is
in open loop. EQ_RAT shall indicate
1.0 while in closed-loop fuel.
Fuel  systems  that  utilize  wide-
range/linear  oxygen  sensors  shall
display the commanded equivalence
ratio in both open-loop and closed-
loop operation.
To obtain the actual A/F ratio be-
ing  commanded,  multiply  the  stoi-
chiometric  A/F  ratio  by  the  equiva-
lence  ratio.  For  example,  for  gaso-
line, stoichiometric is 14.64:1 ratio. If
the  fuel  control  system  was  com-
manding  a  .95  EQ_RAT,  the  com-
manded  A/F  ratio  to  the  engine
would  be  14.64  x  0.95  =  13.9  A/F
ratio.
data are not well represented in the
EQ  data.  The  EQ  ratio  data  looks
almost  backwards  when  compared
to  the  rich  periods  on  the  AFR’s
(low)  and  the  O
2’s  (high).  The  EQ
looks  like  it’s  going  in  the  opposite
direction.
Okay,  now  let’s  look  at  the  point
of  defect.  I  skewed  the  B1S1  AFR
sensor.  You  can  see  the  immediate
skew in the AFR sensor and O
2sen-
sor  data.  The  O
2sensor  rails  at  the
bottom  (lean).  The  AFR  B1S1  ini-
tially  skews  down,  recovers  at  idle
and  then  skews  down  again  under
load.  The  AFR  sensor  is  skewed  to
look  rich,  a  false  signal  I  created.
The fuel response is to react to lean
the “rich” mixture. The rear O
2sen-
sor  shows  the  enleanment  and  the
EQ shows the command to lean.
Is the EQ using just Bank1? Is it
an  average  of  both  banks?  Right
now I have more questions than an-
swers.  I’ll  skew  Bank2  next  time
and see where it leads.
At 105,000
miles, the rear O
2
sensors don’t
go above .8V
and lay flat on
zero for someperiod of time.
DRIVEABILITY CORNER FEB 2015v2_Layout 1  1/22/15  10:22 AM  Page 3

38. 6 Novem ber2014
DéjàVu AllOverAgain
A2007ChevyImpala witha3.5Lenginecame
intoourshopforthefirsttimeaboutayearand
ahalfago.TheMILwason,theenginehadre-
ducedpowerandDTCP0121(ThrottlePosi-
tionSensor1Performance)wasstoredinthe
PCMmemory.Iremoved,cleanedandre-
mountedthethrottlebody,thenflashedthe
PCM andinspected thewiringharness. The
DTCdidnotreturn,sothevehiclewasre-
turnedtothecustom er.About sixmonthslater,
thethrottlebodyfailedwiththesameDTC
P0121stored.Atthistimethethrottle bodywas
replacedwitharemanufactured aftermarket
part.Thewiringharnesswasalsorerouted,as
itdidnotappear tohaveenough slackbetween
thebodyandtheengine.
Fast-forward anothersixmonthsorsoand
thethrottle bodyfailedagain.Thesymptoms
werethesame(reduced power )andthesame
DTCP0121wasstoredinmemory.Ifollowe d
alloftherecommended diagnosticproced ures,
thenreplacedthethrottlebody(asecondtime).
Werecentlyheardfromthecustomer,andthe
vehicleisapparentlyexperiencingtheall-too-
familiarreduced power symptomsandthe
CheckEnginelightison.Isittimetoinstall a
newOEpart?Ihavenotpreviouslyhadany
proble mwithremanpartspurchase dfromthis
supplier.Isthereanunderlyingissuethatis
shorteningthethrottlebody’slifespan?Idon’t
wanttothrowanymoreofthecustomer’smon-
eyatthisproblemwithoutfindingananswe r.
JerryBurns
Trenton,NJ
Duetothelargeamountoftimethathas
elapsed betweeneachfailure occurrence, we’d
havetoconsiderthistobeaveryintermittent
Karl
Seyfert
Onedefinitionofinsanityisrepea tingthesameactionandexpect-
ingadifferent outcome.Aftermultiplereplacementsofthesame
part,itmaybesanertolookelsewhereforthecauseofthefailure.
[email protected]
TroubleShooter
Perhaps duetosafety considerations, this2007 Chevy Impala throttle-by-wire throttle housing has“no
userserviceable parts inside.” Anyaccumulated gunk canberemoved from theareaaround thethrottle
blade, butfamiliar adjustments tocomponents liketheTPsensor arenolonger possible.
Photo:KarlSeyfert
Pg_EDIT_Trouble:Layout 110/21/14 1:42PMPage 1

39. problem.Intermittentsarecertainly
moredifficult,butnotimpossible,to
diagnose.
Perhapsthemosthelpfulinforma-
tionthatcouldbeusedtosolvethis
problemwouldbethefreezeframe
data.Thiswouldtellyoutheoperat ing
conditionsatornearthemomentwhen
theDTCP0121wasstored. Werethe
freezeframedataparametersthesame
(orsimilar) eachtimetheDTCwas
stored?Iftheywere,isitbecausemore
thanonethrottleposition sensorhas
failedinexactlythesameway?This
scenarioisnotimpossible,butitseems
statisticallyunlikely,unlessaseriesof
faultypartswereinvolved.
Ingeneralterms,whatdoweknow
aboutthepossiblecausesofaP0121?It
begins whenthePCM detectsamal-
functionthat’scausinganexcessively
loworhighvoltagesignaltobesent
fromthrottle positionsensortothe
PCM. Thiscanbecaused byathrottle
positionsensorthathasaninternal
fault.Sincethesensorcan’tbereplaced
separately, thisisprobably whyyou’ve
beeninstallingreplacementthrottle
bodyassemblies.TheDTCcanalsobe
causedbyathrottlepositionsensorhar-
nessthat’sopenorshorted. Apooror
intermittentelectricalconnectionin
thethrottlepositionsensorcircuit
couldalsobetoblame.Lastly,and
probablytheleastlikely,thePCMmay
beexperien cingintermittentfailures.
Becausethisisathrottle-by-wire
system,thePCMrespondstoprob-
lemswithitsinputsbyreducingengine
power.Undernormalconditions,the
PCMusestheTPsensorinputtode-
tecttheactualposition ofthethrottle
valve,aswellastheopeningandclos-
ingspeedofthethrottlevalve.Ifthe
TPsensorreportsthatthethrottle
valveisclosed,thePCMwoulduse
thisinformationtocontrolotherfunc-
tions,suchasfuelcut.
IfthePCMdoesnothaveaccurate
informationabouthowfaropenor
closedthethrottlevalvemaybeata
givenmoment,itcan’taccuratelycon-
troltheopeningandclosingofthe
throttlefromthatpointon.There-
ducedenginepowerallowsthedriver
to(barely)limpthecarintoaservice
facility.Thismaybeaninconvenienc e,
butshouldbeconsidered saferthanthe
possibilityofarunawa ythrottle.
When thecustomerbringsthevehi-
cletoyourshopthistime,makecertain
youcapturethefreezeframedatabe-
foremakinganychangestothePCM
oritsprogra mming. When didthe
DTC store?Whatwashappening at
thetime?Withtheoriginalthrottle
housingstillinplace,makeyourbest
attempttoduplicatetheseconditions.
Monitortherelevant PIDswithyour
scantool.Toopenanevenlargerwin-
dowintothisproblem,attachadigital
storageoscilloscopetotheTPsensor’s
datalines.Watchthescopeforanyin-
dicationofsignalabnormalitiesasyou
workthethrottlethroughitsnormal
rangeofmovements.Thismaybea
temperature-relatedfailure,soitmay
benecessarytodrivethevehiclelong
enoughtogeteverythingunderthe
hoodgoodandwarm.
There areafewharness connec tions
between theTPsensor andthePCM.
Examine eachofthemcloselyforany
signsoflooseness,frettingorother
damage.Youmentionedthatthehar-
nessappearedtootightbetweenthe
throttlehousing andthebody.Isitpos-
siblethatthisisorwascausingahar-
nessconnectortopartiallyseparate,
causingtheTPsensor signal toweaken
orintermit tentlydropout?Onceagain,
manipulatingtheharnesswhileobserv -
ingtheTPsensorsignalonthescope
mayallowyoutocaptureanintermit-
tentfailure.
Lastly,there’syourquestionabout
thequalityofthepartsinvolved,and
thepossiblelinktorepeate dfailures.In
myresearch,Ifoundthatthrottlehous-
ingfailuresarenotunheardofonthese
vehicles,sotheoriginalequipment
partscertainlyarenotunbreakable.
Sometechshaveexperiencedproblems
withremanufactured replacement
parts, whileothershavenot.Before
pointingthefinger ofblameatanyre-
placement part,beitoriginalequip-
mentoraftermarket,neworremanu-
factured,I’dsuggest youfirstmakecer-
tainyou’veeliminatedalloftheother
possibl eproblem causes.Installingan-
otherthrottlehousin gwithoutdoingso
might buyyousometime,butdéjà
vucouldstillbeapossibility.
8 Novem ber2014
TroubleShooter
Circle #6
Pg_EDIT_Trouble:Layout 110/21/14 1:43PMPage 2

40. 4 October 2014
Ready toBuyaCat?
IamhavingaproblemwithaDTCP0420that’s
puzzlingme.Thevehicleismy2009CadillacSTS,
whichhasabout85,000milesonit.It’sequipped
witha3.6Ldirectfuelinjection V6engine,asix-
speedautomatictransmissionandAWD.
Thefreezeframedataindicatesbank1wasin
openloopwhentheP0420wasset.Bank2wasin
closedloop.Iclearedthecodeanditcameback
inabout300miles.Onceagain, bank1wasin
openloopwhentheDTCset.Duringsubsequent
testing,bothbank1andbank2wentintoclosed
loopwithinafewminutesafterenginestarts.
TheB1S1oxygensensor switchesnormally,
andresponds tothrottle(wide-open andclosed).
Eventhough itappear edtobefunctioning nor-
mally,IreplacedtheB1S1O
2sensor withanOE
partanyway,thinkingitmightbeanintermittent
O
2sensor. Afterthat,Iclearedthecode,butit
camebackagaininabout700miles.
Icheckedforintakeleaks(withpropane) and
exhaustleaks(bypluggingthetailpipewitharag
andlisteningforasound change). Iwasunableto
findanyleaks.TheBAROreadingof98KPa
seemstobeinagreem entwithwhereIlive
(MetroDetroitarea).
Iamtryingtoruleoutotherpossibleexpla-
nationsfortheP0420beforehavingthefaithto
replacethecat.Whatpuzzles meiswhybank1
isinopenloopwhilebank2isinclosedloop
whenP0420isset.
IsearchedforthemostcommonOBDIIcodes
onanautomotivereferencewebsite.P0420wasat
topofthelist(with13.2% ofthetotal).P0430 was
number10with3.2%ofthetotal.Ican’tthinkof
areasonwhytherearesignificantlymoreP0420s
thanP0430s recorded.Ithinkthisisinteresting.
Itypicallytrytospendenoughtimeonadi-
agnosisuntilIamconfidentaboutmakinga
recommendationtoreplaceanyparts.Butthis
P0420 withbank1inopenlooppuzzlesme.I
haveincludedthefreezeframedatathatwas
storedatthesametimetheDTCwasset.I
wouldappreciateyourhelp.
PushengChen
Novi,MI
AnyDTCthatsetsonlyevery300to700milesis
goingtobetoughtodiagnose. Andonethatpoints
topossible replacement ofanexpensive emissio ns
control component likeacatalytic converterwhen
itdoesisgoingtobeeventougher. Sofirstletme
applaudyourdedication.Andsecond,thankyou
fortheforesighttosaveandincludethefreeze
frame datawithyournote.Thisdatamaynotpro-
videalloftheinformation weneedtoreachadiag-
nostic conclusion, butitshould helptogetus
pointed intherightdirection.
P0420isaverypopularDTC—o runpopular ,
depend ingonhowyoulookatit.Itindicatesthat
thePCMhasdeterm inedthatthecatalyticcon-
verter isperforming below anestablished
threshold. OBDII’snumber onemissionisto
keepvehicleemissions aslowaspossible, andit
can’tdothatwithoutaproperlyfunctio ningcat-
alyticconverter (orconverters, insomecases).
OBDIIkeepsacloseeyeonconvert erperform -
Karl
Seyfert
Decidingtoreplaceanexpensiveemissionscontro lcomponent
requiresconfidence intheaccuracy ofyourdiagnosis.Isthe
decisiontougheroreasierwhen you’reworkingonyourowncar?
[email protected]
TroubleShooter
Freeze frame dataisperhaps themost useful information available when at-
tempting todetermine thecause anOBDIIdiagnostic trouble code. Thisdata
iscollected atthemoment theDTCissetandisthenextbestthing tobeing
there when ithappens. What canthedata shown here tellusabout the
P0420 thatwassetonthe2009Cadillac STSwhen itwascollected?
Freezeframedata:PushengChen
Pg_EDIT_Trouble:Layout 19/25/148:10AMPage 1

41. ance,andwhenperformancedropsbe-
lowaprescribedlevel,thePCMwillset
aDTC.Some mightarguethatper-
formance levelsaresettootightly,mak-
ingitalltooeasyforaconvertertofail
anOBDIImonitor.
Manyvehiclesareequipped with4-
cylinder engines, whichtypicallyhave
justasinglecatalytic converter,orone
largeandonesmallcat,coupledwitha
setofpre-andpostco nverter oxygen
sensorssituatedateither endofthe
maincat.YourSTSisequipped with
twoofeverythingbecauseit’saV6with
separateemissionsequipmentforeach
bank.P0430 pointstoabank2catalytic
converterthat’soperati ngbelowanes-
tablishedthreshold.Many vehicl es
don’thavethissecondconverter,which
IbelieveexplainswhyP0430issomuch
furtherdown onthe“hitparade” of
DTCs,when compared tothechart-
toppingP0420.Ifallvehicleshadtwoof
everything, thetwocatalystefficiency
DTCswouldprobably bemoreevenly
rankedintermsofoccurrence .
ThePCMdoesn’thaveafive-gasex-
haustanalyzerprobestuck upthe
tailpipeofyourSTS,sohowdoesit
makethedeterminati onthatthecon-
verterisfunctioningbelowtheperform-
ancethreshold?ThePCMrunsacata-
lystmonitortestonlywhencertaindriv-
ingconditionshavebeenmet.Theen-
gineandconvert ermustbeatoperating
temperature,andtheengine maybe
idlingorrunningunderlightloadatlow
speed.Yourfreezeframedataindicates
theSTS’sengine speed was1228rpm.
Thefuelsystemshouldalsobein
closed-loopfuelcontrol(thisiskey).
Theremustnotbeanyotherunfulfilled
criteriaorpreviouslystoredDTCs that
wouldkeepthecatalystefficiencymoni-
torfromrunning.
OncethePCMhasdeterminedthat
allpreconditions havebeenmet,ittem-
porarilyforcestheair/fuelmixturerich,
todeplete anystoredoxygeninthecon-
verter.ThenthePCM temporarily
forcestheair/fuelmixtureleantodeter-
minehowlongittakesfortheconverter
toreactandforthedownstreamoxygen
sensortochange itsswitchingactivity.If
theconvertertakestoolongtoresume
functioning(indicatedbypostconverter
oxygensensoractivity), itmeansthecat-
alystisnotworkingefficientlyenoughto
maintainthevehicle’semissions levels
withinprescribedlimits.OBDIIwill
thenfailtheconverter,setaDTCP0420
andturnontheCheckEnginelight.
Ibelieveyourvehicleissettinga
DTCP0420onlyevery300to700miles
becausethat’showlongittakesforallof
thepreconditio nstobemet,andforthe
PCMtorunthecatalystefficienc ymon-
itor.Alternately,themonitormaybe
running morefrequently,andfailing
onlyonceevery300to700miles.
Thekeypieceofinformationcon-
tained inthefreeze frame dataisthein-
dicationthatbank1wasinopenloopat
thetimethefreezeframe datawas
stored.Onthefaceofit,thismakesno
sense,asthecatalystefficienc ymonitor
shouldneverhaveruninthefirstplace
withhalfofthefuelsystem stillinopen
loop.Achieving closedloopisoneofthe
firstpreconditionsthefuelsystem
wouldhavetosatisfybeforethePCM
would evenconside rrunning thecata-
lystefficiency monitor.
Weknowthatfreezeframe datais
storedatthemomen tthePCMdecides
toflagaDTC. Sointhiscasethedata
wasprobably collectedacertain period
oftimeafterthePCMattempted torun
thecatalystefficienc ymonitor .Thefuel
systemhadtobeinclosedloopwhen
themonitor began torun,butsome-
thinghappened afterthat,anditwasno
longerinclosedloopwhenthefreeze
frame datawasstored. Thisisahypoth -
esis,aswedon’tknowhowquickly the
PCM updatesthefreeze frame data
we’renowusingforourdiagno sis.
I’dsuggest youlookforacomponent
that’scapableofintermittentlykicking
thefuelsystem outofclosed loop.This
maybehappeningatothertimes,be-
sideswhenthePCM isattemptingto
runthecatalystefficiency monito r.Be-
sidesthepre-andpostcatalystoxygen
sensors,mostoftheotherinputsensors
havetheirownOBDIIDTCs that
shouldgiveyouanindication ofaprob-
lem.Butitmaybetoointermittentto
triggeraDTCandtheonlywayyou
maybeabletoidentify itisbymonitor -
ingalimited setofPIDs,waitingforthe
glitchtorevealitself.Itcan’thideforev-
er,andyou’vealreadyshownthatyou
havethepatience towait.
7October2014
TroubleShooter
Pg_EDIT_Trouble:Layout 19/24/143:22PMPage 2

42. Gary Stamberger – Training Director
Magnaflow Exhaust Products


In the first part of this OBD II Code Diagnosis series I stated that we would discuss the principles of OBD II codes and
breakdown each character that defines them. For a generic discussion of OBD I’ll refer you to TB-80016 and 80017. We archive all
of our bulletins and they can be found on our website at www.maganaflow.com
. Look for Tech Bulletins under Tech Support. For
this series I would like to stay on a more specific path.

In our first two parts we took a very common Ford EGR code and broke down the diagnosis. I chose this code not only for its
commonality but also because this EGR system uses several components, each one playing a major role in the vehicles ability to
reduce NOx. Although the PCM has the ability to set several different and distinct codes for each component (9 generic and 10
specific) the interrelation of the components cannot be ignored. As we saw in our example, one of the possible causes for the P0401
code was mechanical and had nothing to do with the malfunction of any one component.

Another common issue in Code Diagnostics sometimes overlooked is that of retrieving codes in both OBD II Generic and Enhanced
or Manufacture Specific mode. Depending on the tool being used, the enhanced option may not be available (i.e. Code Reader only).

Using generic mode requires less input therefore is faster and in most cases will get the technician to where he wants to be. The
downside is that it is a generic code and therefore in many cases the repair information will not be specific to that vehicle.

The obvious upside then to using Enhanced Mode, is that the diagnostic information will be specific to that vehicle or at least that
manufacturer. The description and operation will give you a better idea of what the PCM is looking for and the subsequent testing
should lead you to the proper diagnosis the first time.

Example: 2005 Altima, 2.5L with an illuminated MIL. The OBD II code was P0140, O2 Circuit B1S2 No Activity Detected. A quick
glance at the data stream showed that under the proper test conditions the sensor displayed activity. At this point we might determine
that it is an intermittent problem, clear the code and send the customer on their way. However a look at Enhanced codes revealed a
P1147, O2 B1S2 Maximum Voltage not Obtained. A closer look at data stream showed that the sensor was not reaching a specific
maximum voltage of .78v. This specific information was not available when processing the P0140 code.

The key to any diagnostic situation is to always follow a pattern for each problem we face and code diagnostics is no different. Yes…
each manufacture has common problems and knowing where to find that information is valuable but sometimes even the “silver
bullet” can be a dud! Whether it is a no start, misfire, won’t idle, MIL illuminated or any number of issues, having a plan is by far the
best plan. “Shot Gun” diagnosis will on occasion allow us to hit the illusive homerun but more often than not we spend a whole day
repairing a component only to go home with that empty feeling in our stomachs, knowing the same problem will reoccur in the
morning.

Diagnostics is an art and getting good at it can be a great confidence booster, however these vehicles are changing constantly and
there is no time to rest. As I say when closing all my classes:

THE RULES ARE ALWAYS CHANGING
TECHNOLOGY KEEPS MOVING FORWARD
EDUCATION IS A CONTINUAL PROCESS

Cleaning up the environment…one converter at a time

Gary


OBD II Code Diagnosis Part III



Bulletin TB-80035
September, 2011

43. Gary Stamberger – Training Director
Magnaflow Exhaust Products


As promised from last month, more on OBD. Refer to our Website, Magnaflow.com for archived Bulletins.
(http://www.magnaflow.com/07techtips/techbulletins.asp
)

Data Stream

Referred to as Current Data or Live Data, this information is available to the technician using a Scan Tool. The number of PIDS (Parameter Identification)
available at any given time will depend on a couple of different factors. The particular vehicle (Manufacturer) involved will have the greatest influence on the
amount of data available. Followed by the type of Scan Tool used and whether you are viewing the data on the Global OBD II side or Manufacture Specific, aka
Enhanced Mode. (Figure 1) Most Scan Tools will have options for viewing the data in different formats such as digital or graphing mode. Graphing can be
particularly useful when looking at Oxygen Sensor activity. (Figure 2) The data available will consist of inputs and outputs, calculated values and system status
information.

Viewing data and becoming proficient at recognizing problem areas is one of the skills we spoke of in last months Bulletin (TB-80016). Part of any training on a
particular tool is the repetitive process of using it over and over until you begin to recognize when certain data doesn’t look right. This process will then lead you
toward a problem area where further testing will reveal the fault. You can not recognize bad data until you have looked at enough good data. One item to be aware
of is the practice of substituting good data values for suspect ones. Due to something called Adaptive Strategy, when the PCM suspects that a particular input may
not be reporting accurately, it will substitute a known good value for that sensor and run the vehicle on learned values. This will only show up in Enhanced Mode
as Global OBD II will always display actual values. This should not deter you from viewing in Enhanced Mode. It has always been my practice to look at codes
and data in both modes.



FIGURE 1 FIGURE 2

Freeze Frame

Freeze frame is a “snap shot” of data taken when a code is set. This can be very valuable information as it allows the technician an opportunity to duplicate the
conditions under which the trouble code was recorded. The number of freeze frame events recorded and viewable by the technician will again depend on the
vehicle and scan tool being used. Early systems could only store one batch of information, if more than one code was recorded we would typically only be able to
view the Freeze Frame for the last code set. Changes in both OBD and Scan Tool technology have allowed us to have multiple sets of information available for
multiple codes set. One exception is that of Misfire. Misfire codes and subsequent data take precedent and will overwrite any previous freeze data stored. Be
aware that all freeze frame information is lost when codes are cleared.

On Board Diagnostics Part II


Bulletin TB-80017
December, 2009

44. FIGURE 3 FIGURE 4
Courtesy Toyota

Monitors

Monitors, also referred to as Readiness Indicators are considered the single most comprehensive change that came with OBD II. CARB and the EPA
recognized that a vehicle started polluting long before the PCM recognized a fault, set a code and illuminated the MIL. Early OBD systems did not
have the capability to recognize degradation of components or systems. Today’s OBD II system is designed to recognize when a vehicle could
potentially exceed its designed emission standard by a factor of 1.5. It does this through a series of system Monitors.

During normal operation the PCM will conduct certain tests to gauge the operational health of a particular system or component. The Monitors operate
in two categories, Continuous and Non-Continuous. As you can probably guess the Continuous Monitors run, well, continuously. They are Misfire,
Fuel System and Comprehensive Component. Non-Continuous consist of Catalyst, Evaporative, Oxygen Sensor, Oxygen Sensor Heater, EGR
Monitor and more. These require a very specific Drive Cycle (Figure 4) that will meet all the criteria necessary for a complete test. Scan Tools will
have a Monitor Status screen that indicates if the Monitors have run to completion. (Figure 3) Next to each component or system it will indicate
“Ready” or “Not Ready”, “Complete” or “Incomplete”. If the vehicle is not equipped with a certain system the screen will indicate “Not Supported” or
“Not Available”.

When one or more indicators read Not Ready or Incomplete, it is an indication that codes have been cleared recently, either with a scan tool or loss of
power to the PCM such as battery disconnect. If there is no history of either of these events occurring this is an indication of the PCM intermittently
loosing power or it is rebooting which could be an internal problem. It is commonly known that the Catalyst and Evaporative System Monitors are the
hardest to run to completion.

Many states have moved to an OBD system test for Emission Testing in place of tail pipe testing for vehicles 1996 and newer. California is
considering this transition as we move into 2010 (No date has been set for implementation). The test includes checking for proper location of DLC
(Data Link Connector), bulb check of MIL, no MIL when vehicle is running, no codes in system and all the Monitors have run to completion.
Monitors are a key component because they are a direct indication of whether the OBD system had been tampered with prior to Inspection.

The USEPA and CARB authorities have generally found that OBD II systems are more effective in detecting emission-related malfunctions on in-use
vehicles compared to existing Inspection and Maintenance (I/M) tailpipe testing procedures. Current Smog Check data indicates that vehicles are more
likely to fail an OBD II-based inspection than the required tailpipe emissions test. With the reduced testing times (10 mins. for OBD vs. 20 mins. for
tail pipe) and cost savings in equipment it’s not beyond the realm of possibility that states currently having none or minimal Inspection Programs may
consider adopting an OBD Emissions Testing program. These programs have proven to create a healthy environment and also a healthy bottom line
for repair shops.


Cleaning up the environment…one converter at a time

Gary

45. Gary Stamberger – Training Director
Car-Sound/Magnaflow Performance Exhaust

This month we take the discussion of Oxygen Sensors to yet another level. In recent discussions we talked about the role these sensors played in
closed loop fuel control. What exactly does that mean, “Closed loop fuel control”, and what role does it play in maintaining a good working
converter?

When a vehicle is started cold there is a warm up period which is referred to as, “Open loop”. It’s during this time period that the engine is polluting
the most. Consequently, getting to closed loop fuel control is a top priority. The PCM has an internal clock that restarts on each start-up and it knows,
based mainly on temperature, how long before all components are operating and it is ready to enter closed loop. To this end, many elements have been
added to the systems. Oxygen sensors have built in heaters to speed the warm up process. The PCM can detect when the engine is taking too long to
come up to temperature and will set a code P0125, “Insufficient temperature for closed loop fuel control” which typically means the thermostat is
stuck open.

Once the conditions are met and the PCM gains fuel control the goal then becomes maintaining it. The oxygen sensor is referred to as a, “Voltage
Generator” and reports the content of oxygen in the exhaust stream to the PCM ranging between 100mv (Millivolts) and 900mv. When the oxygen
content is high, (Voltage is low, near 100mv) the PCM sees this as a lean condition and its response is to add fuel. When the sensor reports back that
there is little oxygen in the exhaust stream (high voltage, near 900mv), a rich condition is sensed and the PCM pulls fuel away. A technician can
monitor this data on a scan tool as, “Short Term Fuel Trim” or STFT. A positive percentage indicates the computer is adding fuel while a negative
number says it is taking fuel away. If the PCM is in fuel control, monitoring the direct relationship between O2 and STFT scan data will confirm it.



The next step then is to look at Long Term Fuel Trim (LTFT) percentages. These numbers give us a history
of what the PCM has been doing with fuel
trim over the long haul. As with STFT, positive percentages tell us the tendency is to be adding fuel (compensating for a lean condition) while
negative numbers indicate the PCM is pulling fuel back, (Overcoming a rich condition). If either of these conditions exists for a prolonged period of
time and the LTFT percentages exceed the PCM’s parameters a fuel trim code will set (P0170-P0175) and Check Engine light illuminated. The
example below shows us that although the PCM appears to be in fuel control there is evidence that it has been adding fuel over time.


Our concern when looking at fuel trim is what it may be telling us about engine efficiency and whether the computer has been compensating for other
fuel related problems. If the engine has been over-fueling the question is…WHY? A leaking fuel injector, fuel pressure regulator, lazy O2, or bad
Mass Air Flow (MAF) would be some of the considerations. The same issue exists if it’s too lean. Here an air leak, clogged injectors or fuel filter, or
miscalculated air flow could be the cause. Any Fuel Trim condition that persists will eventually take its toll on the catalytic converter and must be
addressed by the repair technician before installing a new one.


Cleaning up the environment…one converter at a time


Gary


INTERPRETING FUEL TRIM DATA



Bulletin TB-80010
May, 2009

53. November 2006 Premier Issue

Practical uses of Mode $06 , by Phil Fournier
Page 1 of 1
Practical uses of Mode $06
Round out your diagnostic skills
By Phil Fournier

54. November 2006 Premier Issue

Practical uses of Mode $06 , by Phil Fournier
Page 2 of 2

As I worked to get a handle on the presentation of Mode 6 at a technician’s level, I was
reminded of a slide in one of my PowerPoint presentations that announces “the lab scope
allows the technician a look inside the manufacturer’s electronic strategy.” I’m going to
make a similar statement here regarding the use of Mode 6. Mode 6 gives the technician a
look inside the manufacturer’s strategy. Sometimes it’s a blurry look, and sometimes it’s
a look of limited usefulness, but taking the look is worth the trouble. It will make the
technician who takes the trouble a better-rounded diagnostician, even if he/she only uses
it for a few selected items.


What It Is
First off we have to cover some basics for those still uncertain of what mode 6 is or isn’t.
Mode 6 is part of the SAE standards that defined what kind of data would be available to
technicians through the OBD2 interface. Simply put, it is the brains behind the operation
of the OBD2 monitors of various emission control systems. In theory, it covers what we
know as the non-continuous monitors, those usually run by the OBD2 system one per trip
if the conditions are right. By now, we all know that those include Fuel Evap, Catalyst,
O2 sensor, O2 sensor heater, EGR, and so forth. But the cool thing about the information
available in some mode 6 data is that it breaks down the monitor into its various parts,
sometimes giving us useful information that cannot be seen as well through looking at
live data stream or looking at stored trouble codes.

I’m going to start off by suggesting that if you are serious about learning the benefits of
Mode 6, invest a few bucks in a scan tool capable of doing the interpretation for you. If
you don’t know what I mean by that, it means you are not currently using Mode 6. The
first time I stumbled across Mode 6 data was while randomly pushing buttons on my scan
tool and looking at stuff. I rapidly backed out of the screen due to what looked to me to
be completely useless information, filled with $ signs, things called TID’s and CID’s,
plus letters and numbers mixed. And so it is unless you have a way to interpret the data.
This is because Mode 6 was written in Hexadecimal code (Base 16 instead of Base 10)
and not particularly designed with the technician in mind. But never mind, there are
plenty of things we have learned to use that fall into this same category.


Hex to Key
Because this article is designed to be useful information, I don’t want to get bogged down
in a boring discussion of Hexadecimal numbers and Base 16. I will just suggest though
that you can use your free Windows calculator to convert the letters and numbers into
pure numbers. If you use “Scientific” from the “View” menu, you will get a choice of the
“Decimal” calculator or the “Hexadecimal” calculator. Entering the letter and number
combination into the Hexadecimal screen and then clicking the button for Decimal will
convert the number to a readable number. Unfortunately, that number will still do you

55. November 2006 Premier Issue

Practical uses of Mode $06 , by Phil Fournier
Page 3 of 3
little good unless you know what it means via a conversion key. Part of the reason that
this article is based on Ford vehicles is because the Ford information is available for free
at www.motorcraftservice.com . Choose “OBD2 Theory and Operation” from the menu
on the left of the screen, then scroll down and pick the Adobe Acrobat file to open. Once
the file is open you can go to the monitor that you want to look at.


Misfire Counts
But leaving all that behind, there is one area of Mode 6 on a Ford which you can start
using immediately, as long as you have some scan tool that will display Mode 6 (and not
all of them do; I’ll include a list later in the article of assorted scan tools and how to find
Mode 6 in them.) I refer to misfire counts, which are not contained in Ford’s regular
Enhanced data stream on the majority of scan tools. Test ID $51 in 1996-98 vehicles and
Test ID $53 from 1997 and on in others will display misfire counts WITH THE
CORRESPONDING CYLINDER shown as the Component ID ($01 through $0A). Note
that $0A is Hex code for the number 10 and indicates cylinder number 10. If you have
less than ten cylinders, you can ignore the data in any of the CID’s above your number of
cylinders because it is bogus. But the beauty of this data is that at last you can identify
which cylinder has misfire counts in it, especially when you have NO CODE. The reason
for this can be seen in the data. Look at the figure below, captured off a 1999 Merc
Marquis with no codes, but a complaint of a misfire under certain load conditions. Note
that I am using the AutoEnginuity PC based Scan tool that interprets the data for us but
you can most likely use your own scanner in a similar fashion.



Notice that all cylinders have zeros in the misfire counts column except cylinder #3. The
counts are low, nowhere near the threshold required to set a code. But this information
was invaluable on a coil-on-plug engine that I had no way to connect to in looking for a
misfire under load condition. After inspecting the plug boot and spark plug for any sign
of arching, and finding none, all I had to do to verify the coil was failing was to swap it
with another cylinder and take the car for a second road test, after clearing codes. (Note:
Though there were no codes, a code clear procedure removes the data from Mode 6
misfire monitors; this is not necessarily true of all data in Mode 6 though.) Finding the
misfire has followed the coil to a different cylinder, the coil can be replaced with perfect
confidence in a proper repair.

56. November 2006 Premier Issue

Practical uses of Mode $06 , by Phil Fournier
Page 4 of 4
It is a bit ironic that Mode 6 data on a Ford contains misfire counts, since SAE J1979
defined Mode 6 as non-continuous monitor information and Mode 7 as continuous.
Never mind, we’ll take the data for its usefulness, even if it is in the wrong place. But this
little snafu is symptomatic of Mode 6 data in general. Not all things are as they might be
expected to be, which makes some technicians throw up their hands and conclude that the
moving target is not worth the trouble. But let’s carry on and see what other use we might
get out of it.


Cat Stats
I know many technicians who sweat over the replacement of a catalyst because of a code
P0420/P0430. When factory cats cost in excess of $800 each, it is small wonder that they
worry about a misdiagnosis. But how about if we could record the Mode 6 data, clear the
code, and then drive the car to see what resets in the Mode 6 monitor? Let’s see what our
1999 Grand Marquis with 108k miles on it shows for Mode 6 data on its catalyst monitor.



This cat monitor was run after the coil was replaced, and we can safely conclude by
looking at the data that this catalyst is still in good condition. The switch ratio of the bank
1 cat (the side of the misfiring coil) is actually a bit better than that of bank 2. And both
cats are well under the limit described here as .842, which according to the Ford website
reflects a limit of the percentage of switching of the rear O2 sensor as opposed to the
front one. Notice what happens to the data when we clear codes:



It didn’t go to zero, did it? Instead, some random number got put in the box, a number
that looks like a near-failure of .749. But what’s the likelihood that both cats would
measure the exact same switch ratio? Just about zero, but this illustrates the need to not
be careless in your treatment of Mode 6 information. The graphic below (captured from a
1998 Ford Winstar) show what the data would like uninterpreted:


See Chart on following page!

57. November 2006 Premier Issue

Practical uses of Mode $06 , by Phil Fournier
Page 5 of 5


You could do the math, converting the numbers from Hex to Decimal, and then
multiplying by the conversion factor listed on the website of .0156. But why bother? It is
easy to see that the two catalysts on this vehicle are well within the maximum limit.
But what if you had a code P0420, cleared it, then drove the vehicle and saw the 10/11
test number at 19? You could be pretty comfortable at recommending the catalyst, and
depending on where you saw the 10/21 parameter, you might be recommending a pair.







On the following pages… Restricted EGR and scan tool tips

58. November 2006 Premier Issue

Practical uses of Mode $06 , by Phil Fournier
Page 6 of 6
Choked off?
Restricted EGR is one other place where Mode 6 on a Ford can come in handy. Here is
data from a normal system (1999 Ford Crown Vic 4.6L with no problems):



Notice that the TID $45/$20 is the same as the DPFE voltage as long as the EGR valve is
not stuck open. This can help a technician that may not have enhanced Ford data, as the
generic data stream (Mode 1) does not list DPFE voltage as a parameter.

Listed below is the data (same vehicle) where I disconnected the intake side hose on the
inconveniently located DPFE (see photo ?):



Note that this set a pending code P0401(EGR flow insufficient) on a single road test.
However, because we have no specs for TID 41/CID11 & 12, they do not help us like
they are supposed to and the vehicle does not set the code P1405 like it is supposed to.
But obviously something is detected. Next I reconnected the DPFE hose and installed a
restriction in the EGR valve (see figure ?) to simulate a restricted EGR passage. I
captured the following data:



I find it very interesting that TID $4A/$30 has barely passing numbers, but in order to
achieve that much flow, the computer had to ratchet up the EVR duty cycle to 89%.
However, in spite of repeated road tests and completed EGR monitors, this condition
would not set even a pending code. Below is another capture with the EGR blocked
completely:

59. November 2006 Premier Issue

Practical uses of Mode $06 , by Phil Fournier
Page 7 of 7
Taken all together, we can see that Mode 6 stored data can help us nail down a restricted
EGR passage that does not set a code, now that we know to look at TIDs $4A/$30 and
$4B/30. You can probably do that on your scan tool without an interpreter, but you are
going to have to do the Hex plus conversion math and use the Ford website.

Conclusion
In conclusion, Mode 6 is a barrel of data, some of it bogus and meaningless, and much of
it powerful. I’m told that Honda recommends the use of Mode 6 data in the diagnosis of
fuel evaporative problems. I’d recommend you dive into your scan tool and pick some
TID’s and CID’s to figure out so you can get started on learning what to expect,
particularly if you have the good fortune to work on a single car line. If you are a
multicar line guy like me, try it out on Ford misfires to start with and see if you don’t find
it to be a real time saver.


How to access Mode $06 in assorted scan tools
(found under Generic or Global OBD II in every case):

MasterTech: Select “F5 System Test” then “F2 Other Results” (Note that results are
displayed as “Pass/Fail”. To get the actual readings press “*, Help”)

Snap On: (later than 2001 cartridge, earlier versions don’t have Mode 6): After
communicating with vehicle select “Display Test Parameters/Results” then select “Non-
Continuous Monitored systems (Mode $6). (MT2500; Solus, Modis similar)

BDM: Select “Non-Continuous Monitor Test Results”

NGS: Select Diag Monitoring Test Results

AutoEnginuity: Select On Board Test Results tab

OTC Genesis: Select “Special Tests” then “Component Parameters”





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