Computers and On-Board Diagnostics

Computers and On-Board Diagnostics
Computers and
On-Board Diagnostics
OBJECTIVES: After studying Chapter 26, you should
be able to:
1. Prepare for the interprovincial Red Seal certification
examination in Appendix VIII (Engine Performance)
on the topics covered in this chapter.
2. Explain the purpose, function and operation of "flash"
3. Describe the diagnostic procedures and routines
relating to a trouble code.
4. Explain the purpose and operation of a scan tool.
5. Describe the differences between OBD I and OBD II.
6. Describe how the powertrain control module
performs active and passive tests of the
computerized engine control system.
7. Describe the standardized OBD II DTCs and
8. Explain the purpose behind one- and two-trip logic.
Figure 26–1 A typical malfunction indicator lamp (MIL),
often labelled “Check Engine” or “Service Engine Soon.”
On-Board Diagnostics:
Early Systems
During the 1980s, many manufacturers began
equipping their vehicles with full-function control
systems capable of alerting the driver of a malfunction and of allowing the technician to retrieve codes
that identify circuit faults. These early diagnostic
systems were meant to reduce emissions and assist
the technician. The automotive industry calls these
systems on-board diagnostics (OBD).
The powertrain control module (PCM) has a built-in
self-diagnosis program that detects failures or major
faults in the engine management system and alerts
the driver by illuminating a Malfunction Indicator
Lamp (MIL). The MIL informs the driver to “Check
Engine,” “Service Engine Soon,” or “Power Loss.” See
Figure 26–1.
The lamp will stay on if the problem is present
(hard fault) and will go out if the problem no longer
exists (soft fault). A fault code will set and remain in
computer memory for approximately 25 to 30 engine
starts (most vehicles). This is an aid to the technician
when diagnosing the system.
Figure 26–3 The data link connector on many Asian and
domestic vehicles (non–OBD II) will cause the malfunction
indicator lamp to flash trouble codes when the designated
terminals are connected with a jumper wire. (Courtesy
Toyota Canada Inc.)
Figure 26–2 The data link connector (DLC) is located
under the dash on this General Motors vehicle. It is known
as the assembly-line communications link (ALCL) on early
GM vehicles because it allowed the assembly plant to test
engine operations before the vehicle left the factory. It is
used by service technicians in the field to access trouble
codes and read live data stream. (Courtesy General
Motors of Canada Ltd.)
Fault codes (diagnostic trouble codes—DTCs)
are accessed through a diagnostic (data link connector—DLC) found in many different locations, e.g.,
under the hood, under the dash, in the console or the
glove box. Often, the shop manual must be consulted
for the exact location. See Figure 26–2. The DLC also
varies in appearance among makes.
Flash Codes
The procedures for retrieving DTCs differs among
makes. Many on-board computer diagnostics are entered by connecting two or more terminals in the DLC
with a jumper (GM and many imports); see Figure
26–3. Chrysler cycles the ignition key a given number
of times within 5 seconds. This will activate the MIL,
which begins to flash; count the number of flashes.
Voltmeters are used with some Ford and Mitsubishi vehicles to identify trouble codes. Connecting
a voltmeter into the system, as shown in Figure 26–4,
will cause the meter needle to rise and fall; counting
the number of needle sweeps will identify the DTC.
Ford vehicles also go through a self-test, which
checks the sensors and actuators before giving out
trouble codes.
Figure 26–4 Analog voltmeters are used by Ford and
some import vehicles to read diagnostic trouble codes.
Counting the number of needle sweeps (pulses) will
determine the code. (Courtesy Ford Motor Co. of
Canada Ltd.)
Computers and On-Board Diagnostics
Figure 26–5 Typical list of early 1990s diagnostic trouble codes (DTC). It is important to use the
shop manual (or data bank) when checking codes as they are different between car makers. Onboard diagnostics, generation II (OBD II) standardized most trouble code numbers and
terminology. (Courtesy General Motors of Canada Ltd.)
Diagnostic Trouble Codes
Clearing Trouble Codes
Trouble codes, known previously as fault codes,
are usually listed in numerical order to identify
the circuit. See Figure 26–5. The technician is
then instructed to follow a set of diagnostic routines related to the trouble code. See Figures 26–6
and 26–7.
Trouble codes are cleared from computer memory by
disconnecting a jumper wire, removing a fuse, or
through the use of a scan tool. Refer to the shop manual since procedures differ. Disconnecting the vehicle
battery to clear codes is not recommended, as this will
also erase any adaptive strategy program changes
Figure 26–6 Typical diagnostic early 1990s flow chart for a DTC. This section of the chart gives the
circuit description, wiring schematic, and diagnostic aids. (Courtesy General Motors of Canada Ltd.)
stored in the computer. Other components such as the
radio, which uses battery power to retain memory, will
also lose their settings.
Scan Tools
Scanners are small hand-held computers that provide a major improvement over flash-code diagnostics. They typically plug into the data link connector
and interface with the on-board computer. Power to
operate the scanner is supplied through the lighter
socket or a battery adaptor; late-model OBD II scanners receive power at the DLC. See Figure 26–8.
Scanners have the ability to read directly from live
data stream; information from the input sensors and
output actuators may be monitored during a road test.
Many scan tools have a snap-shot mode, which allows
the technician to freeze certain data at the point the
Computers and On-Board Diagnostics
Figure 26–7 Typical diagnostic flow chart. This section takes the technician, step by step, through
the diagnostic routines. (Courtesy General Motors of Canada Ltd.)
driveability concern arrives. This information can then
be reviewed and interpreted back in the service bay.
The majority of Asian and European vehicles have
no provisions for live data stream readouts with
early on-board diagnostics. Today, virtually every automobile sold in Canada and the U.S. is equipped to
provide running data.
Scanners also supply trouble code information in
numerical form; there are no light flashes or needle
sweeps to count.
Some scanners have the ability to control the output actuators and solenoids for test purposes. Performing a cylinder-balance test by interrupting the ignition spark is a common diagnostic routine used with
Figure 26–8 Hand-held scan tools interface with the onboard computer. They not only extract fault codes (DTC),
they read live data from the sensors and actuators. Prior
to OBD II, the scan tool required a different DLC adaptor
and program cartridge when switching between makes.
(Courtesy Toyota Canada Inc.)
many large oscilloscopes. This must be done very carefully, as the fuel and air from the dead cylinder will
flow into the catalytic converter, causing it to overheat.
Feeding the converter raw fuel and oxygen causes internal catalyst temperatures to rise quickly and the
converter will begin to melt if the cylinder is "killed"
for long. Vehicles with sequential (individual injector
control) fuel injection often use scanners to cancel each
fuel injector, instead of ignition, for cylinder balance
testing. This protects the converter and eliminates any
chance of a backfire in the exhaust pipes.
Many late vehicles have no provision for flashcode retrieval and a scanner must be used to extract
trouble codes.
On-Board Diagnostics:
Generation I (OBD I)
The California Air Resources Board (CARB) developed the first regulation requiring manufacturers
selling vehicles in that state to install OBD. Called
OBD Generation I (OBD I), OBD I applies to all vehicles sold in California beginning with the 1988
model year. It carries the following requirements:
1. An instrument panel warning lamp able to
alert the driver of certain control system
failures, now called a malfunction indicator
lamp (MIL).
2. The system's ability to record and transmit
diagnostic trouble codes (DTCs) for
emission-related failures.
3. Electronic system monitoring of the HO2S, EGR
valve, and evaporative purge solenoid. Although
not EPA-required during this time, most
manufacturers also equipped vehicles sold
outside of California with OBD I.
These initial regulations failed to meet many expectations. By failing to monitor the catalytic converter, the
evaporative system for leaks, and the presence of engine misfire, OBD I did not do enough to lower automotive emissions. In addition, the OBD I monitoring
circuits that were installed lacked sufficient sensitivity.
Aside from OBD I's lack of emission-reduction
effectiveness, another problem existed. Auto manufacturers implemented OBD I rules as they saw fit,
resulting in a vast array of servicing tools and systems. Rather than simplifying the job of locating
and repairing a failure, the aftermarket technician
faced a tangled network of procedures often requiring the use of expensive special test equipment and
dealer-proprietary information.
Soon it became apparent that more stringent
measures were needed if the ultimate goal, reduced
automotive emission levels, was to be achieved. This
led to the development of OBD Generation II (OBD II).
OBD II Objectives
Generally an OBD II vehicle is defined by its ability to:
1. Detect component degradation or a faulty
emission-related system that prevents
compliance with federal emission standards.
2. Alert the driver of needed emission-related
repair or maintenance.
3. Use standardized DTCs and accept a generic
scan tool.
OBD II was first introduced on some 1994 vehicles;
by 1998, all light-duty vehicles sold in Canada (U.S.
1996) were required to be OBD II compliant. The primary purpose of OBD II is emission-related,
whereas the primary purpose of OBD I (1988) was
to detect faults in sensors or sensor circuits. OBD II
regulations require that not only must the sensors
be tested but that all exhaust control devices be
tested and verified for proper operation.
All new vehicles must pass the Federal Test
Procedure (FTP) for exhaust emissions while being tested for 505 seconds on rollers that simulate
the urban drive cycle around downtown Los Angeles,
NOTE: IM 240 is simply a shorter version of the 505second-long federal test procedure.
The regulations for OBD II vehicles state that the
vehicle computer must be capable of testing for exhaust emissions, and determining whether or not they
are within 1 1/2 times the allowable standard for a new
vehicle based on the FTP limits. In order to achieve
this goal, the computer has to do all of the following:
Computers and On-Board Diagnostics
1. Test all exhaust emission system components
for correct operation.
2. Actively operate the system and measure the
3. Continuously monitor all aspects of the engine
operation to be certain that the exhaust
emissions do not exceed 1 1/2 times the FTP.
4. Check engine operation for misfire.
5. Turn on the malfunction indicator lamp (MIL)
(check engine) if the computer senses a fault in a
circuit or system.
6. Flash the MIL if an engine misfire occurs that
could damage the catalytic converter.
Comprehensive Component
The comprehensive component monitor (CCM)
is an internal program in the PCM designed to monitor a failure in any electronic component or circuit
(including emission-related and non-emission-related
circuits) that provide input or output signals to the
PCM. The PCM considers that an input or output signal is inoperative when a failure exists due to an open
circuit, out-of-range value or if an on-board rationality check fails. If an emission-related fault is detected, the PCM will set a code and activate the MIL
(requires two consecutive trips). Some exceptions are
(a) serious engine misfire that could damage the catalytic converter—this requires one trip only; (b) catalyst monitoring which requires three trips.
Many PCM sensors and output devices are tested
at key on or immediately after engine startup. However, some devices, such as the idle air control (IAC),
are only tested by the CCM after the engine meets
certain engine conditions. The number of times the
CCM must detect a fault before it will activate the
MIL depends upon the manufacturer, but most require two consecutive trips to activate the MIL. The
components tested by the CCM include:
4-wheel-drive low switch
Brake switch
Camshaft (CMP) and crankshaft (CKP) sensors
Clutch switch (manual transmissions/transaxles
Cruise servo switch
Engine coolant temperature (ECT) sensor
EVAP purge sensor or switch
Fuel composition sensor
Intake air temperature (IAT) sensor
Knock sensor (KS)
Manifold absolute pressure (MAP) sensor
Mass airflow (MAF) sensor
Transmission fluid temperature (TFT) sensor
Transmission turbine speed sensor
Vacuum sensor
Vehicle speed (VS) sensor
EVAP canister purge and EVAP purge vent
Idle air control solenoid
Ignition control system
Transmission torque converter clutch solenoid
Transmission shift solenoids
Main Monitors
On OBD II systems, the PCM incorporates a special
segment of software. This software program is designed to manage the operation of all OBD II monitors by controlling the sequence of steps necessary to
execute the diagnostic tests and monitors:
Comprehensive component monitor
Catalyst monitor
EGR and EVAP system monitors
Fuel system monitor
Misfire monitor
Oxygen sensor monitor
Oxygen sensor heater monitor
Secondary AIR system monitor
A list of devices or systems tested by OBD II
comprehensive component monitor (CCM) and main
monitors includes the devices in the following table.
Component Monitor
Main Monitors
BARO, ECT, and IAT sensor
Fuel control system (fuel trim)
MAF, MAP, or MDP sensors
Misfire detection
Oxygen sensor—voltage
level, activity
Catalyst efficiency
CMP, CKP, and TP sensors
EGR system
EGR, EVAP solenoids
EVAP system
Idle speed control motor
Oxygen sensor—response
Fuel injectors
Oxygen sensor heater
Some PCM switches
Secondary AIR system
NOTE: The number of trips required by the CCM and
main monitors before a code is set and the MIL is activated varies among vehicle manufacturers.
See Figures 26-9 to 26-11.
OBD II Drive Cycle
The vehicle must be driven under a variety of operating conditions for all active tests to be performed. OBD
II regulations also established a vehicle “drive cycle”
pattern that would allow the CCM and main monitors
to run and complete their individual diagnostic tests.
Figure 26–9 Fuel system monitor. The exhaust-gas oxygen sensor monitors the air-fuel ratio (in closed loop) and signals
the on-board computer. If the mixture is incorrect, the computer adds or subtracts fuel to bring the mixture into range.
This happens constantly and is known as short-term fuel trim. When short-term fuel trim is always rich (or lean), long-term
fuel trim shifts from its original program and adjusts fuel delivery to bring the air-fuel mixture again back into range. If the
correction needed reaches a pre-set limit, the MIL will illuminate. (Courtesy Ford Motor Co.)
Figure 26–10 The misfire monitor
reduces emissions and protects the
catalytic converter. When a misfire
occurs, raw hydrocarbons and unburned
oxygen are pumped into the converter,
raising internal temperatures. (a) The
crankshaft position sensor (CKP), also
used for ignition, sends a signal to the
PCM for each pulse ring tooth. (b) A
misfire will cause the crankshaft to slow.
The increased time between pulses
indicates a misfire. The fuel injector for
the offending cylinder may be shut off by
the PCM. (Courtesy Ford Motor Co.)
Computers and On-Board Diagnostics
Figure 26–11 Catalyst
efficiency. The efficiency of
the catalytic converter(s) is
determined by: (a) placing
oxygen sensors before and
after the converter; (b)
comparing the downstream
sensor reading with the
upstream signal. If a
malfunction is detected on
three drive cycles, the MIL
is illuminated. (Courtesy
Ford Motor Co.)
The OBD II monitors that should run during the drive
cycle include the CCM, EGR, EVAP, Fuel System, Misfire, Oxygen Sensor, and Secondary AIR System. One
Frequently Asked Question
What Does “Rationality Check” Mean?
The power train control module (PCM) is programmed to
detect faults that do not seem rational. For example, if the
engine has been operating for 20 minutes and suddenly the
engine coolant temperature changes from 90°C (195°F) to
40°C (40°F), then the rationality test part of the computer program (CCM) determines that this is not possible
(rational) and then defaults to a fail-safe operating temperature based largely on the intake air temperature (IAT) sensor. Before OBD II regulations, if the engine coolant temperature sensor became unplugged, the computer would
increase the amount of fuel delivered to the engine because
it assumed that the engine was in fact very cold. With rationality, the OBD II computer can reason that there must
be a fault and continue to deliver fuel for proper operation
and not too much,which could affect the exhaust emissions.
manufacturer has a special code (Ford—DTC P1000)
that sets if all the main monitors have not been run to
A trip is defined as an engine-operating drive
cycle that contains the necessary conditions for a
particular test to be performed. These conditions
are called the enable criteria. For example, for the
EGR test to be performed, the engine has to be at
normal operating temperature and decelerating for
a minimum amount of time. Some tests are performed when the engine is cold, whereas others require that the vehicle be cruising at a steady highway speed.
Warm-Up Cycle
The MIL will deactivate (turn off) if the PCM no
longer detects a fault during three consecutive trips
(warm-up cycles). Once a MIL is deactivated, the
original code will remain in memory until 40 warmup cycles are completed without the fault reappearing. A warm-up cycle is defined as a trip with an engine temperature increase of at least 22°C (40°F)
and where engine temperature reaches at least
70°C (160°F).
MIL Condition: Off
This condition indicates that the PCM has not detected any faults in an emissions-related component
or system, or that the MIL circuit is not working.
MIL Condition: On Steady
This condition indicates a fault in an emissionsrelated component or system that could affect the
vehicle emission levels.
MIL Condition: Flashing
This condition indicates a misfire or fuel control system fault that could damage the catalytic converter.
A vehicle is driven on three consecutive trips
with a warm-up cycle and meets all code set
conditions without the PCM detecting any faults.
The PCM will set a code if a fault is detected that
could cause tailpipe emissions to exceed 1 1/2 times
the FTP standard. However, the PCM will not deactivate the MIL until the vehicle has been driven on
three consecutive trips with vehicle conditions similar to actual conditions present when the fault was
detected. This is not merely three vehicle start-ups
and trips. It means three trips where certain engine
operating conditions are met so that the OBD II
monitor that found the fault can run again and pass
the diagnostic test.
Fuel Trim and Misfire Codes
NOTE: In a misfire condition with the MIL on steady,
if the driver reaches a vehicle speed and load condition
with the engine misfiring at a level that could cause
catalyst damage, the MIL would start flashing. It
would continue to flash until engine speed and load
conditions caused the level of misfire to subside. Then
the MIL would go back to the on steady condition. This
situation might result in a customer complaint of a
MIL with an intermittent flashing condition.
MIL: Off
The PCM will turn off the MIL if any of these actions
or conditions occur:
The codes are cleared with a scan tool.
Power to the PCM is removed at the battery or
with the PCM power fuse for an extended period
of time (may be up to several hours or longer).
Frequently Asked Question
If a fuel control system (fuel trim) or misfire-related
code sets, then the vehicle must be driven under
conditions similar to when the fault was detected
before the PCM will deactivate the MIL. Similar
conditions are:
The vehicle must be driven with engine speed
within 375 rpm of the engine speed stored in the
freeze-frame data when the code set.
The vehicle must be driven within engine load
10% of the engine load value stored in the
freeze-frame data when the code set.
The vehicle must be driven with engine
temperature conditions similar to the
temperature value stored in freeze-frame data
when the code set.
See Figure 26–12.
How Can All the Readiness Tests Be Set?
Readiness tests (sometimes called flags) are tests performed on all of the monitored systems as displayed on a scan tool.
To run all tests, the engine coolant temperature should be less than 50°C (122°F) with the IAT within 6°C (11°F) of the
ECT temperature and the fuel tank filled from 15% to 85% of capacity before starting the test. Proceed as follows:
1. Start the engine and allow it to idle for 2 1/2 minutes. This step tests the oxygen sensor heater, canister purge system,
misfire, fuel trim, and time to closed loop operation.
2. Accelerate at half throttle to 90 km/h (55 mph). This step tests for misfire, fuel trim diagnostics, and canister purge.
3. Hold the speed steady for 3 minutes. This step tests the oxygen sensor, EGR system, canister purge, misfire, and fuel
trim diagnostics.
4. Decelerate without using the brake or clutch (if equipped). This step tests the EGR system, canister purge, and fuel trim
5. Accelerate at three-fourths throttle to 90 to 100 km/h (55 to 60 mph). This step tests for misfire, fuel trim diagnostics, and
canister purge.
6. Hold steady speed for 5 minutes. This step tests the catalytic converter.
7. Decelerate without using the brake or clutch, if equipped. This step tests the EGR system, canister purge, and fuel trim
Computers and On-Board Diagnostics
TRIP (Run Monitors)
Figure 26–12 How a PCM turns on the MIL.
First failure
Type B—First failure of
Type B (two-trip) fault on
this key cycle that is not a
Fuel Problem or Misfire
Type B—First failure of
Type B (two-trip) for Fuel
Problem or Misfire will
arm DTC and run Monitor
for next 80 nonconsecutive trips
Type A—First failure
of Type A (one-trip)
fault on this key cycle
Type B—Second
consecutive failure of Type
B (two-trip) fault that is
not Fuel Problem or
Type B—Second nonconsecutive Fuel Problem
or Misfire failure under
similar conditions in next
80 trips
Request MIL on and write
Freeze Frame—Stores
Figure 26–13 A typical scan tool
set up for OBD II diagnostics. Ford
calls this a New Generation SelfTest Automatic Readout tester (NG
STAR or NGS). Most OBD II scan
tools receive power through the
DLC and do not require a separate
power connector. (Courtesy Ford
Motor Co.)
Retrieving OBD II Codes—16 Pin
A scan tool is required to retrieve DTCs from an OBD
II vehicle. See Figure 26–13. Every OBD II scan tool
will be able to read all generic Society of Automotive
Engineers (SAE) DTCs from any vehicle. See Figures
26–14 and 26–15 for definitions and explanations of
OBD alphanumeric DTCs. Although all data link connectors are the same on OBD II vehicles, some manufacturer-discretion pins are used for different purposes.
See Figure 26–16. Generic scan tools often supply "personality keys" (small adaptors that fit into the scan tool
plug) to match the pin use for the vehicle being tested.
DTC Numbering Explanation
The number in the hundredth position indicates the
specific vehicle system or subgroup that failed. This
position should be consistent for P0xxx and P1xxx
Figure 26–14 An alphanumeric DTC chart for
OBD II. SAE (generic) codes are standardized
for all vehicles. The example P1711 indicates a
transmission oil temperature circuit out of
range. (Courtesy Ford Motor Co.)
Figure 26–15 Sixteen-pin OBD II DLC with
terminals identified. Scan tools use the power
(#16) pin and ground (#4) pin so that a
separate cigarette lighter plug is not necessary
on OBD II vehicles.
type codes. The following numbers and systems were
established by SAE:
P0100—air metering and fuel system fault
P0200—fuel system (fuel injector only) fault
P0300—ignition system or misfire fault
P0400—emission control system fault
P0500—idle speed control, vehicle speed sensor
P0600—computer output circuit (relay, solenoid,
etc.) fault
P0700—transaxle, transmission faults
NOTE: The tens and ones numbers indicate the part
of the system at fault.
Computers and On-Board Diagnostics
OBD II Active Tests
The vehicle computer must run tests on the various
emission-related components and turn on the malfunction indicator lamp (MIL). OBD II is an active
computer analysis system because it actually tests
the operation of the oxygen sensors, exhaust gas recirculation system, and other systems whenever conditions permit. It is the purpose and function of the
powertrain control module (PCM) to monitor these
components and perform these active tests.
For example,the PCM may open the EGR valve momentarily to check its operation while the vehicle is decelerating. A change in the manifold absolute pressure
(MAP) sensor signal will indicate to the computer that
the exhaust gas is, in fact, being introduced into the engine. Because these tests are active and certain conditions must be present before these tests can be run, the
computer uses its internal diagnostic program to keep
track of all the various conditions and to schedule active tests so that they will not interfere with each other.
Types of DTCs
Not all OBD II DTCs are of the same importance for
exhaust emissions. Each type of DTC has different
requirements for it to set, and the computer will only
turn on the MIL for emissions-related DTCs.
Type A Codes A type A DTC is emission-related
and will cause the MIL to be turned on on the first
trip if the computer has detected a problem. Engine
misfire or a very rich or lean air–fuel ratio, for example, would cause a type A DTC. These codes alert
the driver to an emission problem that may cause
damage to the catalytic converter.
Type B Codes A type B code will be stored and
the MIL will be turned on during the second consecutive trip, alerting the driver to the fact that a diagnostic test was performed and failed.
NOTE: Type A and B codes are emission-related codes
that will cause the lighting of the malfunction indicator lamp, usually labelled “check engine” or “service engine soon.”
Type C and D Codes Type C and D codes are for
use with non-emission-related diagnostic tests;
they will cause the lighting of a “service” lamp (if
the vehicle is so equipped). Type C codes are also
called type C1 codes, and D codes are also called
type C0 codes.
OBD II Freeze-Frame
To assist the service technician, OBD II requires the
computer to take a “snapshot” or freeze-frame of all
data at the instant an emission-related DTC is set. A
scan tool is required to retrieve this data.
What Are Each of the Pins for in the OBD II 16-Pin Data Link Connector (DLC)?
All OBD II vehicles use a 16-in connector that includes:
Ford vehicles use:
Pin 4 chassis ground
Pin 5 signal ground
Pin 16 battery power (4A max)
• SAE J-1850(PWM) (PWM - 41.6 kB) standard, which uses
pins 2, 4, 5, 10, and 16
Vehicles may use one of two major standards including:
• ISO 9141-2 Standard (ISO International Standards
Pins 7 and 15 (or wire at pin 7 and no pin at 2 or a wire at 7
and at 2 and/or 10)
• SAE J-1850 Standard (SAE Society of Automotive
Two types: VPW (variable pulse width) or PWM (pulse
width modulated)
Pins 2 and 10 (no wire at pin 7)
DaimlerChrysler, European, and Asian vehicles use:
• ISO 9141-2 standard, which uses pins 4, 5, 7, 15, and 16
• DaimlerChrysler Domestic OBD II
Pins 2 and 10—CCM
Pins 3 and 14—OEM Enhanced—60 500 baud rate
Pins 7 and 15—Generic OBD II—ISO 9141—10 400 baud rate
Figure 26–16 OBD II DLC pin use.
• Ford Domestic OBD II
Pins 2 and 10—CCM
Pins 6 and 14—OEM Enhanced—Class C—40 500 baud rate
Pins 7 and 15—Generic OBD II—ISO 9141—10 400 baud rate
General Motors vehicles use:
• SAE J-1850 (VPW - Class 2 - 10.4 kB) standard, which uses
pins 2, 4, 5, and 16 and not 10
• GM Domestic OBD II
Pins 1 and 9—CCM (Comprehensive Component Monitor)
slow baud rate—8192 UART
Pins 2 and 10—OEM Enhanced—Fast Rate—40 500 baud rate
Pins 7 and 15—Generic OBD II—ISO 9141—10 400 baud rate
NOTE: Although OBD II requires that just one freezeframe of data be stored, the instant an emission-related
DTC is set, vehicle manufacturers usually provide expanded data about the DTC beyond that required.
However, to retrieve this enhanced data usually requires the use of the vehicle-specific scan tool.
Freeze-frame items include:
Calculated load value
Engine speed (RPM)
Short-term and long-term fuel trim percent
Fuel system pressure (on some vehicles)
Vehicle speed (km/h or mph)
Engine coolant temperature (ECT)
Intake manifold pressure
Closed/open loop status
Fault code that triggered the freeze-frame
If a misfire code is set, identify which cylinder is
Don’t Forget—Three Clicks
OBD II requires that the fuel system integrity be checked
for possible leakage. If the fuel (gas) cap is not securely
tightened, then a DTC such as P0442 may be set. To help
prevent such false codes and to ensure that the gas cap is
properly tightened, General Motors Corporation has
printed on the cap itself a note that the cap should be
tightened until three clicks are heard. This also applies to
other screw-thread-type gas caps of all years and makes
to be assured that the cap is tight. Some vehicles are
equipped with an amber “Check Gas Cap” lamp that will
light if a system leak is detected.
NOTE: Gas caps are frequently tested as part of an
exhaust emission test. Ask the person performing the
test on your gas cap to tighten the cap three clicks to
ensure proper tightness. This will help prevent false
defective test results.
Clearing OBD II DTCs
A DTC should not be cleared from the vehicle computer memory unless the fault has been corrected and
the technician is so directed by the diagnostic procedure. If the problem that caused the DTC to be set has
been corrected, the computer will automatically clear
the DTC after 40 consecutive warm-up cycles with no
further faults detected (misfire and excessively rich or
lean condition codes require 80 warm-up cycles). The
codes can also be erased by using a scan tool.
NOTE: Disconnecting the battery may not erase OBD
II DTCs or freeze-frame data. Most vehicle manufacturers recommend using a scan tool to erase DTCs
rather than disconnecting the battery because the
memory for the radio, seats, and learned engine operating parameters are lost if the battery is disconnected.
Frequently Asked Question
What Are Pending Codes?
Pending codes are set when operating conditions are met
and the component or circuit is not within the normal
range, yet the conditions have not yet been met to set a
DTC. For example, a sensor may require two consecutive
faults before a DTC is set. If a scan tool displays a pending
code or a failure, a driveability concern could also be present. The pending code can help the technician try to determine the root cause before the customer complains of a
check engine light indication.
Diagnostic Procedures
Diagnostic procedures for OBD I and OBD II vehicles are covered in Chapter 31, “Engine Performance
Diagnosis and Testing.”
If a computer fails, it is often difficult to determine if
the computer itself is at fault or if there is a problem
with some other system in the vehicle. For example,
if the engine stalls, it could be the result of a fault in
the ignition system, fuel system, or a failed sensor
such as a crankshaft position sensor (CKP).
As part of the diagnostic process, check the computer grounds as shown in Figure 26–17. Also gently
Figure 26–17 Always check that the computer grounds
are clean and tight.
Computers and On-Board Diagnostics
1. List four components that are tested by the comprehensive component monitor (CCM).
2. What is the difference between a warm-up cycle and a
3. What is a pending code?
4. What are “flash” codes?
5. How are codes cleared from PCM memory?
1. All vehicles sold in Canada since _____ must be
equipped with OBD II.
a. 1996
b. 1998
c. 2000
d. 2002
2. The primary purpose of OBD I is _____.
a. Emission related
b. Fuel injection control
c. To detect faults in sensors or circuits
d. Improving fuel economy
3. A loose gas cap can set a diagnostic trouble code
a. True
b. False
4. OBD II DTC PO172 (System Too Rich) will automatically clear from memory after _____ warm-up cycles
when the problem is corrected.
a. 2
b. 3
c. 40
d. 80
Figure 26–18 Tap testing a vehicle computer. General
Motors recommends that only the four fingers of an open
hand be used to tap test any component to avoid causing
5. A warm-up cycle has to achieve at least how many degrees of engine coolant temperature?
a. 15ºC (60ºF)
b. 50ºC (122ºF)
c. 70ºC (160ºF)
d. 80ºC (177ºF)
6. Which DTC represents an ignition or misfire fault?
a. P0100
b. P0200
c. P0300
d. P0400
tap on the computer with the engine running. If the
engine stalls or changes the way it is operating,
check the wiring connecter. If the wiring is OK, replace the computer. See Figure 26–18.
1. Malfunction indicator lamp (MIL) is the name given to
the amber check engine or “service engine soon” light.
2. On-board diagnostics second generation, called OBD
II, is used on all vehicles sold in Canada since 1998.
3. OBD II requires that all emission-related components
be checked and tested.
4. The vehicle must be driven with approximately the
same speed, load, and temperature (similar conditions) before the PCM will deactivate the MIL for fuel
trim and misfire codes.
5. The data link connector (DLC) and generic diagnostic
trouble codes (DTC) are the same for all OBD II vehicles.
7. An ignition misfire or fuel mix problem is an example
of what type of DTC?
a. A
b. B
c. C
d. D
8. A type B DTC requires how many faults to turn on
the MIL?
a. One
b. Two
c. Three
d. Four
9. A freeze-frame is generated on an OBD II vehicle _____.
a. Whenever a type C or D diagnostic trouble
code is set
b. Whenever a type A or B diagnostic trouble
code is set
c. Every other trip
d. Whenever the PCM detects a problem with
the O2S
10. Terminal 16 of the OBD II DLC supplies volts to a scan
tool, and terminal ______ supplies the signal ground.
a. 2
b. 5
c. 4
d. 1
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