26

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"
codes.
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 Figure 26–1 A typical malfunction indicator lamp (MIL),
terminology. often labelled “Check Engine” or “Service Engine Soon.”
8. Explain the purpose behind one- and two-trip logic.

On-Board Diagnostics:
Early Systems
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
ON-BOARD DIAGNOSTICS the driver by illuminating a Malfunction Indicator
Lamp (MIL). The MIL informs the driver to “Check
During the 1980s, many manufacturers began Engine,” “Service Engine Soon,” or “Power Loss.” See
equipping their vehicles with full-function control Figure 26–1.
systems capable of alerting the driver of a malfunc- The lamp will stay on if the problem is present
tion and of allowing the technician to retrieve codes (hard fault) and will go out if the problem no longer
that identify circuit faults. These early diagnostic exists (soft fault). A fault code will set and remain in
systems were meant to reduce emissions and assist computer memory for approximately 25 to 30 engine
the technician. The automotive industry calls these starts (most vehicles). This is an aid to the technician
systems on-board diagnostics (OBD). when diagnosing the system.
621
622 CHAPTER 26

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 connec-
tor—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 en-
tered 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 Mit-
subishi 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 Figure 26–4 Analog voltmeters are used by Ford and
the number of needle sweeps will identify the DTC. some import vehicles to read diagnostic trouble codes.
Ford vehicles also go through a self-test, which Counting the number of needle sweeps (pulses) will
checks the sensors and actuators before giving out determine the code. (Courtesy Ford Motor Co. of
trouble codes. Canada Ltd.)
 Computers and On-Board Diagnostics 623

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. On-
board 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, Trouble codes are cleared from computer memory by
are usually listed in numerical order to identify disconnecting a jumper wire, removing a fuse, or
the circuit. See Figure 26–5. The technician is through the use of a scan tool. Refer to the shop man-
then instructed to follow a set of diagnostic rou- ual since procedures differ. Disconnecting the vehicle
tines related to the trouble code. See Figures 26–6 battery to clear codes is not recommended, as this will
and 26–7. also erase any adaptive strategy program changes
624 CHAPTER 26

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 and interface with the on-board computer. Power to
radio, which uses battery power to retain memory, will operate the scanner is supplied through the lighter
also lose their settings. socket or a battery adaptor; late-model OBD II scan-
ners receive power at the DLC. See Figure 26–8.
Scanners have the ability to read directly from live
Scan Tools data stream; information from the input sensors and
Scanners are small hand-held computers that pro- output actuators may be monitored during a road test.
vide a major improvement over flash-code diagnos- Many scan tools have a snap-shot mode, which allows
tics. They typically plug into the data link connector the technician to freeze certain data at the point the
 Computers and On-Board Diagnostics 625

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 Scanners also supply trouble code information in
be reviewed and interpreted back in the service bay. numerical form; there are no light flashes or needle
The majority of Asian and European vehicles have sweeps to count.
no provisions for live data stream readouts with Some scanners have the ability to control the out-
early on-board diagnostics. Today, virtually every au- put actuators and solenoids for test purposes. Per-
tomobile sold in Canada and the U.S. is equipped to forming a cylinder-balance test by interrupting the ig-
provide running data. nition spark is a common diagnostic routine used with
626 CHAPTER 26

These initial regulations failed to meet many expecta-

tions. By failing to monitor the catalytic converter, the
evaporative system for leaks, and the presence of en-
gine misfire, OBD I did not do enough to lower auto-
motive 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 manu-
facturers implemented OBD I rules as they saw fit,
resulting in a vast array of servicing tools and sys-
tems. Rather than simplifying the job of locating
and repairing a failure, the aftermarket technician
faced a tangled network of procedures often requir-
ing the use of expensive special test equipment and
Figure 26–8 Hand-held scan tools interface with the on- dealer-proprietary information.
board computer. They not only extract fault codes (DTC), Soon it became apparent that more stringent
they read live data from the sensors and actuators. Prior measures were needed if the ultimate goal, reduced
to OBD II, the scan tool required a different DLC adaptor automotive emission levels, was to be achieved. This
and program cartridge when switching between makes. led to the development of OBD Generation II (OBD II).
(Courtesy Toyota Canada Inc.)
OBD II Objectives
many large oscilloscopes. This must be done very care-
fully, as the fuel and air from the dead cylinder will Generally an OBD II vehicle is defined by its ability to:
flow into the catalytic converter, causing it to overheat.
Feeding the converter raw fuel and oxygen causes in- 1. Detect component degradation or a faulty
ternal catalyst temperatures to rise quickly and the emission-related system that prevents
converter will begin to melt if the cylinder is "killed" compliance with federal emission standards.
for long. Vehicles with sequential (individual injector 2. Alert the driver of needed emission-related
control) fuel injection often use scanners to cancel each repair or maintenance.
fuel injector, instead of ignition, for cylinder balance 3. Use standardized DTCs and accept a generic
testing. This protects the converter and eliminates any scan tool.
chance of a backfire in the exhaust pipes.
Many late vehicles have no provision for flash- OBD II was first introduced on some 1994 vehicles;
code retrieval and a scanner must be used to extract by 1998, all light-duty vehicles sold in Canada (U.S.
trouble codes. 1996) were required to be OBD II compliant. The pri-
mary purpose of OBD II is emission-related,
On-Board Diagnostics: whereas the primary purpose of OBD I (1988) was
to detect faults in sensors or sensor circuits. OBD II
Generation I (OBD I) regulations require that not only must the sensors
The California Air Resources Board (CARB) devel- be tested but that all exhaust control devices be
oped the first regulation requiring manufacturers tested and verified for proper operation.
selling vehicles in that state to install OBD. Called All new vehicles must pass the Federal Test
OBD Generation I (OBD I), OBD I applies to all ve- Procedure (FTP) for exhaust emissions while be-
hicles sold in California beginning with the 1988 ing tested for 505 seconds on rollers that simulate
model year. It carries the following requirements: the urban drive cycle around downtown Los Angeles,
California.
1. An instrument panel warning lamp able to
alert the driver of certain control system
failures, now called a malfunction indicator NOTE: IM 240 is simply a shorter version of the 505-
lamp (MIL). second-long federal test procedure.
2. The system's ability to record and transmit
diagnostic trouble codes (DTCs) for
emission-related failures. The regulations for OBD II vehicles state that the
3. Electronic system monitoring of the HO2S, EGR vehicle computer must be capable of testing for ex-
valve, and evaporative purge solenoid. Although haust emissions, and determining whether or not they
not EPA-required during this time, most are within 1 1/2 times the allowable standard for a new
manufacturers also equipped vehicles sold vehicle based on the FTP limits. In order to achieve
outside of California with OBD I. this goal, the computer has to do all of the following:
 Computers and On-Board Diagnostics 627

1. Test all exhaust emission system components ■ EVAP canister purge and EVAP purge vent
for correct operation. solenoid
2. Actively operate the system and measure the ■ Idle air control solenoid
results. ■ Ignition control system
3. Continuously monitor all aspects of the engine ■ Transmission torque converter clutch solenoid
operation to be certain that the exhaust ■ Transmission shift solenoids
emissions do not exceed 1 1/2 times the FTP.
4. Check engine operation for misfire.
Main Monitors
5. Turn on the malfunction indicator lamp (MIL)
(check engine) if the computer senses a fault in a On OBD II systems, the PCM incorporates a special
circuit or system. segment of software. This software program is de-
6. Flash the MIL if an engine misfire occurs that signed to manage the operation of all OBD II moni-
could damage the catalytic converter. tors by controlling the sequence of steps necessary to
execute the diagnostic tests and monitors:
Comprehensive Component ■ Comprehensive component monitor
Monitor ■ Catalyst monitor
■ EGR and EVAP system monitors
The comprehensive component monitor (CCM)
■ Fuel system monitor
is an internal program in the PCM designed to mon-
■ Misfire monitor
itor a failure in any electronic component or circuit
■ Oxygen sensor monitor
(including emission-related and non-emission-related
■ Oxygen sensor heater monitor
circuits) that provide input or output signals to the
■ Secondary AIR system monitor
PCM. The PCM considers that an input or output sig-
nal is inoperative when a failure exists due to an open A list of devices or systems tested by OBD II
circuit, out-of-range value or if an on-board rational- comprehensive component monitor (CCM) and main
ity check fails. If an emission-related fault is de- monitors includes the devices in the following table.
tected, 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 cat- Comprehensive
alytic converter—this requires one trip only; (b) cata- Component Monitor Main Monitors
lyst monitoring which requires three trips. BARO, ECT, and IAT sensor Fuel control system (fuel trim)
Many PCM sensors and output devices are tested
MAF, MAP, or MDP sensors Misfire detection
at key on or immediately after engine startup. How-
ever, some devices, such as the idle air control (IAC), Oxygen sensor—voltage Catalyst efficiency
are only tested by the CCM after the engine meets level, activity
certain engine conditions. The number of times the CMP, CKP, and TP sensors EGR system
CCM must detect a fault before it will activate the EGR, EVAP solenoids EVAP system
MIL depends upon the manufacturer, but most re-
quire two consecutive trips to activate the MIL. The Idle speed control motor Oxygen sensor—response
components tested by the CCM include: time
Fuel injectors Oxygen sensor heater
■ 4-wheel-drive low switch
■
Some PCM switches Secondary AIR system
Brake switch
■ Camshaft (CMP) and crankshaft (CKP) sensors
■ Clutch switch (manual transmissions/transaxles
only) NOTE: The number of trips required by the CCM and
■ Cruise servo switch main monitors before a code is set and the MIL is acti-
■ Engine coolant temperature (ECT) sensor vated varies among vehicle manufacturers.
■ EVAP purge sensor or switch
■ Fuel composition sensor See Figures 26-9 to 26-11.
■ Intake air temperature (IAT) sensor
■ Knock sensor (KS)
■ Manifold absolute pressure (MAP) sensor
OBD II Drive Cycle
■ Mass airflow (MAF) sensor The vehicle must be driven under a variety of operat-
■ Transmission fluid temperature (TFT) sensor ing conditions for all active tests to be performed. OBD
■ Transmission turbine speed sensor II regulations also established a vehicle “drive cycle”
■ Vacuum sensor pattern that would allow the CCM and main monitors
■ Vehicle speed (VS) sensor 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
(a)
the offending cylinder may be shut off by
the PCM. (Courtesy Ford Motor Co.)

(b)

628
 Computers and On-Board Diagnostics 629

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.)

(a)

(b)

The OBD II monitors that should run during the drive manufacturer has a special code (Ford—DTC P1000)
cycle include the CCM, EGR, EVAP, Fuel System, Mis- that sets if all the main monitors have not been run to
fire, Oxygen Sensor, and Secondary AIR System. One completion.
A trip is defined as an engine-operating drive
cycle that contains the necessary conditions for a
Frequently Asked Question ??? 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
What Does “Rationality Check” Mean?
normal operating temperature and decelerating for
The power train control module (PCM) is programmed to a minimum amount of time. Some tests are per-
detect faults that do not seem rational. For example, if the formed when the engine is cold, whereas others re-
engine has been operating for 20 minutes and suddenly the quire that the vehicle be cruising at a steady high-
engine coolant temperature changes from 90°C (195°F) to way speed.

40°C (
40°F), then the rationality test part of the com-
puter program (CCM) determines that this is not possible
(rational) and then defaults to a fail-safe operating tempera-
Warm-Up Cycle
ture based largely on the intake air temperature (IAT) sen- The MIL will deactivate (turn off) if the PCM no
sor. Before OBD II regulations, if the engine coolant tem- longer detects a fault during three consecutive trips
perature sensor became unplugged, the computer would (warm-up cycles). Once a MIL is deactivated, the
increase the amount of fuel delivered to the engine because original code will remain in memory until 40 warm-
it assumed that the engine was in fact very cold. With ra- up cycles are completed without the fault reappear-
tionality, the OBD II computer can reason that there must ing. A warm-up cycle is defined as a trip with an en-
be a fault and continue to deliver fuel for proper operation gine temperature increase of at least 22°C (40°F)
and not too much,which could affect the exhaust emissions. and where engine temperature reaches at least
70°C (160°F).
630 CHAPTER 26

MIL Condition: Off ■ A vehicle is driven on three consecutive trips

with a warm-up cycle and meets all code set
This condition indicates that the PCM has not de-
conditions without the PCM detecting any faults.
tected any faults in an emissions-related component
or system, or that the MIL circuit is not working. The PCM will set a code if a fault is detected that
could cause tailpipe emissions to exceed 1 1/2 times
MIL Condition: On Steady the FTP standard. However, the PCM will not deac-
tivate the MIL until the vehicle has been driven on
This condition indicates a fault in an emissions-
three consecutive trips with vehicle conditions simi-
related component or system that could affect the
lar to actual conditions present when the fault was
vehicle emission levels.
detected. This is not merely three vehicle start-ups
and trips. It means three trips where certain engine
MIL Condition: Flashing operating conditions are met so that the OBD II
This condition indicates a misfire or fuel control sys- monitor that found the fault can run again and pass
tem fault that could damage the catalytic converter. 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 If a fuel control system (fuel trim) or misfire-related
with the engine misfiring at a level that could cause code sets, then the vehicle must be driven under
catalyst damage, the MIL would start flashing. It conditions similar to when the fault was detected
would continue to flash until engine speed and load before the PCM will deactivate the MIL. Similar
conditions caused the level of misfire to subside. Then conditions are:
the MIL would go back to the on steady condition. This
situation might result in a customer complaint of a ■ The vehicle must be driven with engine speed
MIL with an intermittent flashing condition. 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
MIL: Off
10% of the engine load value stored in the
The PCM will turn off the MIL if any of these actions freeze-frame data when the code set.
or conditions occur: ■ The vehicle must be driven with engine
temperature conditions similar to the
■ The codes are cleared with a scan tool. temperature value stored in freeze-frame data
■ Power to the PCM is removed at the battery or when the code set.
with the PCM power fuse for an extended period
of time (may be up to several hours or longer). See Figure 26–12.

Frequently Asked Question ???

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
diagnostics.
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
diagnostics.
 Computers and On-Board Diagnostics 631

TRIP (Run Monitors)

Figure 26–12 How a PCM turns on the MIL.
First failure Type B—First failure of Type B—First failure of
Type B (two-trip) fault on Type B (two-trip) for Fuel
this key cycle that is not a Problem or Misfire will
Fuel Problem or Misfire arm DTC and run Monitor
for next 80 non-
consecutive trips
Type A—First failure
of Type A (one-trip)
fault on this key cycle

Type B—Second Type B—Second non-

consecutive failure of Type consecutive Fuel Problem
B (two-trip) fault that is or Misfire failure under
not Fuel Problem or similar conditions in next
Misfire 80 trips

Request MIL on and write

Freeze Frame—Stores
DTC

Figure 26–13 A typical scan tool

set up for OBD II diagnostics. Ford
calls this a New Generation Self-
Test 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 con-
nectors are the same on OBD II vehicles, some manu-
facturer-discretion pins are used for different purposes.
See Figure 26–16. Generic scan tools often supply "per-
sonality 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
632 CHAPTER 26

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

■ P0600—computer output circuit (relay, solenoid,
established by SAE:
etc.) fault
■ P0100—air metering and fuel system fault ■ P0700—transaxle, transmission faults
■ P0200—fuel system (fuel injector only) fault
■ P0300—ignition system or misfire fault
■ P0400—emission control system fault NOTE: The tens and ones numbers indicate the part
■ P0500—idle speed control, vehicle speed sensor of the system at fault.
fault
 Computers and On-Board Diagnostics 633

OBD II Active Tests trip if the computer has detected a problem. Engine
misfire or a very rich or lean air–fuel ratio, for ex-
The vehicle computer must run tests on the various
ample, would cause a type A DTC. These codes alert
emission-related components and turn on the mal-
the driver to an emission problem that may cause
function indicator lamp (MIL). OBD II is an active
damage to the catalytic converter.
computer analysis system because it actually tests
the operation of the oxygen sensors, exhaust gas re-
Type B Codes A type B code will be stored and
circulation system, and other systems whenever con-
the MIL will be turned on during the second consec-
ditions permit. It is the purpose and function of the
utive trip, alerting the driver to the fact that a diag-
powertrain control module (PCM) to monitor these
nostic test was performed and failed.
components and perform these active tests.
For example,the PCM may open the EGR valve mo-
mentarily to check its operation while the vehicle is de- NOTE: Type A and B codes are emission-related codes
celerating. A change in the manifold absolute pressure that will cause the lighting of the malfunction indica-
(MAP) sensor signal will indicate to the computer that tor lamp, usually labelled “check engine” or “service en-
the exhaust gas is, in fact, being introduced into the en- gine soon.”
gine. Because these tests are active and certain condi-
tions must be present before these tests can be run, the Type C and D Codes Type C and D codes are for
computer uses its internal diagnostic program to keep use with non-emission-related diagnostic tests;
track of all the various conditions and to schedule ac- they will cause the lighting of a “service” lamp (if
tive tests so that they will not interfere with each other. the vehicle is so equipped). Type C codes are also
called type C1 codes, and D codes are also called
Types of DTCs type C0 codes.
Not all OBD II DTCs are of the same importance for
exhaust emissions. Each type of DTC has different OBD II Freeze-Frame
requirements for it to set, and the computer will only
turn on the MIL for emissions-related DTCs. To assist the service technician, OBD II requires the
computer to take a “snapshot” or freeze-frame of all
Type A Codes A type A DTC is emission-related data at the instant an emission-related DTC is set. A
and will cause the MIL to be turned on on the first 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 • SAE J-1850(PWM) (PWM - 41.6 kB) standard, which uses
Pin 5  signal ground pins 2, 4, 5, 10, and 16
Pin 16  battery power (4A max)
• Ford Domestic OBD II
Vehicles may use one of two major standards including:
Pins 2 and 10—CCM
• ISO 9141-2 Standard (ISO  International Standards Pins 6 and 14—OEM Enhanced—Class C—40 500 baud rate
Organization) Pins 7 and 15—Generic OBD II—ISO 9141—10 400 baud rate
Pins 7 and 15 (or wire at pin 7 and no pin at 2 or a wire at 7
General Motors vehicles use:
and at 2 and/or 10)
• SAE J-1850 Standard (SAE  Society of Automotive • SAE J-1850 (VPW - Class 2 - 10.4 kB) standard, which uses
Engineers) pins 2, 4, 5, and 16 and not 10
Two types: VPW (variable pulse width) or PWM (pulse
width modulated) • GM Domestic OBD II
Pins 2 and 10 (no wire at pin 7) Pins 1 and 9—CCM (Comprehensive Component Monitor)
DaimlerChrysler, European, and Asian vehicles use: slow baud rate—8192 UART
Pins 2 and 10—OEM Enhanced—Fast Rate—40 500 baud rate
• ISO 9141-2 standard, which uses pins 4, 5, 7, 15, and 16 Pins 7 and 15—Generic OBD II—ISO 9141—10 400 baud rate
• 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.

634 CHAPTER 26

NOTE: Although OBD II requires that just one freeze-

frame of data be stored, the instant an emission-related
T E C H T I P ✔
DTC is set, vehicle manufacturers usually provide ex-
panded data about the DTC beyond that required. Don’t Forget—Three Clicks
However, to retrieve this enhanced data usually re- OBD II requires that the fuel system integrity be checked
quires the use of the vehicle-specific scan tool. for possible leakage. If the fuel (gas) cap is not securely
tightened, then a DTC such as P0442 may be set. To help
Freeze-frame items include: prevent such false codes and to ensure that the gas cap is
properly tightened, General Motors Corporation has
■ Calculated load value printed on the cap itself a note that the cap should be
■ Engine speed (RPM) tightened until three clicks are heard. This also applies to
■ Short-term and long-term fuel trim percent other screw-thread-type gas caps of all years and makes
■ Fuel system pressure (on some vehicles) to be assured that the cap is tight. Some vehicles are
■ Vehicle speed (km/h or mph) equipped with an amber “Check Gas Cap” lamp that will
■ Engine coolant temperature (ECT) light if a system leak is detected.
■ Intake manifold pressure
■ Closed/open loop status NOTE: Gas caps are frequently tested as part of an
■ Fault code that triggered the freeze-frame exhaust emission test. Ask the person performing the
■ If a misfire code is set, identify which cylinder is test on your gas cap to tighten the cap three clicks to
misfiring ensure proper tightness. This will help prevent false
defective test results.
Clearing OBD II DTCs
A DTC should not be cleared from the vehicle com-
puter memory unless the fault has been corrected and
the technician is so directed by the diagnostic proce- Frequently Asked Question ???
dure. If the problem that caused the DTC to be set has
been corrected, the computer will automatically clear
What Are Pending Codes?
the DTC after 40 consecutive warm-up cycles with no
further faults detected (misfire and excessively rich or Pending codes are set when operating conditions are met
lean condition codes require 80 warm-up cycles). The and the component or circuit is not within the normal
codes can also be erased by using a scan tool. 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
NOTE: Disconnecting the battery may not erase OBD
II DTCs or freeze-frame data. Most vehicle manufac- code or a failure, a driveability concern could also be pres-
turers recommend using a scan tool to erase DTCs ent. The pending code can help the technician try to deter-
rather than disconnecting the battery because the mine the root cause before the customer complains of a
memory for the radio, seats, and learned engine oper- check engine light indication.
ating parameters are lost if the battery is disconnected.

Diagnostic Procedures
Diagnostic procedures for OBD I and OBD II vehi-
cles are covered in Chapter 31, “Engine Performance
Diagnosis and Testing.”

DIAGNOSING COMPUTER
PROBLEMS
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 com- Figure 26–17 Always check that the computer grounds
puter grounds as shown in Figure 26–17. Also gently are clean and tight.
 Computers and On-Board Diagnostics 635

REVIEW QUESTIONS

1. List four components that are tested by the compre-

hensive component monitor (CCM).
2. What is the difference between a warm-up cycle and a
trip?
3. What is a pending code?
4. What are “flash” codes?
5. How are codes cleared from PCM memory?

RED SEAL CERTIFICATION-

TYPE QUESTIONS
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
(DTC).
a. True b. False
4. OBD II DTC PO172 (System Too Rich) will automati-
cally 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 5. A warm-up cycle has to achieve at least how many de-
Motors recommends that only the four fingers of an open grees of engine coolant temperature?
hand be used to tap test any component to avoid causing a. 15ºC (60ºF) b. 50ºC (122ºF)
damage. c. 70ºC (160ºF) d. 80ºC (177ºF)
6. Which DTC represents an ignition or misfire fault?
a. P0100 b. P0200
c. P0300 d. P0400
7. An ignition misfire or fuel mix problem is an example
tap on the computer with the engine running. If the of what type of DTC?
engine stalls or changes the way it is operating, a. A b. B
check the wiring connecter. If the wiring is OK, re- c. C d. D
place the computer. See Figure 26–18. 8. A type B DTC requires how many faults to turn on
the MIL?
SUMMARY a. One b. Two
c. Three d. Four
1. Malfunction indicator lamp (MIL) is the name given to 9. A freeze-frame is generated on an OBD II vehicle _____.
the amber check engine or “service engine soon” light. a. Whenever a type C or D diagnostic trouble
code is set
2. On-board diagnostics second generation, called OBD
b. Whenever a type A or B diagnostic trouble
II, is used on all vehicles sold in Canada since 1998.
code is set
3. OBD II requires that all emission-related components c. Every other trip
be checked and tested. d. Whenever the PCM detects a problem with
4. The vehicle must be driven with approximately the the O2S
same speed, load, and temperature (similar condi- 10. Terminal 16 of the OBD II DLC supplies volts to a scan
tions) before the PCM will deactivate the MIL for fuel tool, and terminal ______ supplies the signal ground.
trim and misfire codes. a. 2 b. 5
5. The data link connector (DLC) and generic diagnostic c. 4 d. 1
trouble codes (DTC) are the same for all OBD II vehicles.