Canyon, Colorado 2006-2007

OBD II Systems


The California Air Resources Board (CARB) began regulating On-Board Diagnostic (OBD) systems for vehicles sold in California beginning with the 1988 model year. The initial requirements, known as OBD I, required the identification of the likely area of a fault with regard to the fuel metering system, EGR system, emission-related components and the PCM. Implementation of this new vehicle emission control monitoring regulation was done in several phases.

Class II Serial Data

OBD II regulations require that all vehicle manufacturers establish a common communications system called Class II data. Each bit of information in Class II data can have one or two lengths, long or short. This feature allows a Scan Tool to access communication data from any make or model vehicle sold with Class II data capability.

This feature allows vehicle wiring to carry multiple signals over a single wire. Messages carried on Class II data streams can be prioritized. If two messages attempt to establish communication on the data line at the same time, only the message with the higher priority will continue. The message with the lower priority must wait.

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Fig. Class II Data Example Graphic

Diagnostic Executive

The Diagnostic Executive is a unique segment of software in the PCM designed to coordinate and prioritize the diagnostic procedures as well as define the protocol for recording and displaying their results. The main responsibilities of the Diagnostic Executive are listed below:

Command the MIL either On or Off
Log and clear a diagnostic trouble code (DTC)
Record the Freeze Frame data for the first emission related DTC
Control any non-emission related Service Lamps
Control the operating conditions buffer in the Failure Records
Display current status information on each diagnostic function
Update and display the current I/M Readiness Status "flags"

A key function of the Diagnostic Executive is to record codes and turn on the MIL when an emission related fault is detected. It can also turn off the MIL if the Conditions: go away that caused the code to set.

The PCM continually tests the operation of certain engine control functions. This diagnostic capability is complimented by the diagnostic procedures contained in repair manuals. The language of communicating the source of the fault detected occurs through a set of trouble codes. When a fault is detected by the PCM, a code is set and the MIL is illuminated for all emission related trouble codes.

Diagnostic Test Modes

The "test mode" messages available on a Scan Tool are listed below:

Mode $01: Used to display Powertrain Data (PID data)
Mode $02: Used to display any stored Freeze Frame data
Mode $03: Used to request any trouble codes stored in memory
Mode $04: Used to request that any trouble codes be cleared
Mode $05: Used to monitor the Oxygen sensor test results
Mode $06: Used to monitor Non-Continuous Monitor test results
Mode $07: Used to monitor the Continuous Monitor test results
Mode $08: Used to request control of a special test (EVAP Leak)
Mode $09: Used to request vehicle information (INFO MENU)

Differences Between OBD I & OBD II

As with OBD I, if an emission related problem is detected on a vehicle with OBD II, the MIL is activated and a code is set. However, that is the only real similarity between these systems. OBD II procedures that define emissions component and system tests, code clearing and drive cycles are more comprehensive than tests in the OBD I system.

GM Repair Information

GM repair charts are organized by section in their GM publications. In most cases Section 'A' includes the PCM power and ground circuit tests, No MIL, No DLC Communication, No Start Tests and the Code Repair Tables. Section 'B' includes Driveability Symptoms. Section 'C' includes Component checks of various PCM systems (i.e., Emission Controls, Fuel, Speed Control and Ignition systems). Diagnosis of the OBD II system on GM vehicles starts with an OBD System Check.

Passive & Active Diagnostic Tests

A passive test is a diagnostic test that simply monitors a vehicle component or system. Conversely, an active test actually takes some sort of action when performing diagnostic functions, often in response to a failed passive test. For example, the EGR active test will force the EGR valve open during closed throttle deceleration and may not force the EGR valve closed during a steady state mode. Either action should result in a change in manifold pressure on the MAP sensor.

Intrusive Diagnostic Tests

An intrusive test refers to a test controlled by the PCM as part of OBD II that may have an effect on vehicle performance or emission levels.

Powertrain Control Module

PCM "Learning Capability-

The PCM includes some learning capability that allows it to make corrections for minor variations to improve driveability. If power to the PCM is disconnected, the learning process resets and begins all over again. Most vehicles will relearn operating parameters in a short driving period. Most Scan Tools have a special test mode that can be used on some 1989 and newer models to allow the "relearn" steps.

PCM Hardware & Software

As with earlier GM systems, the PCM is divided into two main parts: the system hardware and software. The system hardware includes:

All related actuators, relays, and solenoids
All related sensors and switches
All interconnecting wires, connectors and terminals

The Power Control Module (PCM)

System software includes programs that contain strategies used by the PCM to control engine system outputs based on related inputs. These are the strategies used to control the operation of the engine, electronic transmission, idle speed, fuel delivery control, and backup circuitry that is used if a major failure occurs inside the PCM.

Purpose Of Onboard Diagnostics

The purpose of Onboard Diagnostics is to provide optimum control of the engine and transmission while meeting the objectives of the OBD II regulations. At the center of this system is a PCM connected to input and output devices via a wiring harness. The PCM receives information from sensors and switches, performs calculations based on data stored in memory and controls various output devices.

System Monitors

Catalyst Monitor

EPA regulations require that the onboard diagnostics must monitor the catalyst once per trip. The catalyst is considered degraded if the hydrocarbon levels at the tailpipe exceed the FTP standards.

The OBD II Catalyst Monitor measures oxygen storage capacity. To test the catalyst efficiency, oxygen sensors are installed before and after the Three-Way Catalyst. Voltage variations between the oxygen sensors allow the PCM to determine the performance of the catalyst.

The Catalyst Monitor test is based on the correlation conversion efficiency and oxygen storage capacity of the catalyst. An efficient catalyst will show a relatively flat output voltage on the post-catalyst heated oxygen sensor. If the catalyst is degraded, the signal from the post-catalyst HO2S will show increased activity (refer to the Graphic).

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Fig. Catalyst Monitor Graphic

Catalyst Degradation & Failure

When a catalyst DTC sets, remember that the converter may have internal or external damage. A few items to check are shown below:

Check for any exhaust leaks (they can hide a degraded catalyst)
Check for contaminated fuel and for miss-matched O2 sensors
Check for the use of alternate fuels and for any aftermarket parts

Comprehensive Component Monitor

OBD II regulations require that the PCM monitor all components and systems that could cause tailpipe emissions to exceed 1.5 times the FTP Standard. The Comprehensive Component Monitor (CCM) is designed to continuously monitor components for a short-to-ground, open circuit or short-to-power condition. It also conducts rationality checks of input devices and functionality checks of output devices.

ECT Sensor Monitor

The ECT Monitor is an on-board diagnostic designed to test the ECT Sensor input for out-of-range conditions. It is also used to detect how long it takes for the engine coolant to reach closed loop temperature.

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Fig. ECT Sensor Wiring Graphic

ECT Closed Loop Enable Test

The Closed Loop Enable Test measures the amount of engine run time required for the ECT sensor input to reach closed loop threshold.

ECT Sensor Out-Of-Range Test

The ECT sensor out-of-range test monitors the temperature reading from the ECT sensor approximately every 100 milliseconds. For a fixed interval of time during the test, the PCM counts the number of ECT sensor inputs outside of its expected range. If the number of ECT sensor inputs in a high or low range exceeds a stored threshold, the ECT Sensor Monitor determines the sensor has failed high or low.

If a relatively small number of samples fall into the high range, DTC P1115 is set (ECT Sensor Intermittent High Input). If the samples fall into the low range, DTC P1114 is set (ECT Sensor Intermittent Low Input). This intermittent test is not required by the OBD II legislation. Refer to Page 3-21 for the OBD II Drive Cycle that includes this test.

EGR System Monitor

Government regulations require that the onboard diagnostics must monitor the EGR system for abnormally high or low EGR flow rates. This system is considered to have failed if a fault is detected that could change the EGR flow rate to a level below the FTP standards.

The EGR system lowers NOx emission levels caused by high engine combustion temperatures. The EGR system accomplishes this task by feeding measured amounts of exhaust gases back into the combustion chamber to change the A/F ratio and dilute the exhaust gases. This action lowers the combustion chamber temperatures.

EGR System Monitor Tests

In most cases, the EGR System Monitor detects changes in manifold pressure during EGR valve actuation to determine if the system is operating effectively. During the test, the EGR solenoid is used to force the EGR valve open during closed throttle deceleration and/or force the valve closed during steady state cruise. Manifold vacuum should increase when the valve is opened during deceleration and the vacuum should decrease when the EGR valve is closed during cruise conditions. In both cases, the amount of change in the MAP sensor input correlates to the amount of EGR flow through the valve.

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Fig. EGR System Graphic

Engine Misfire With Misfire Relief

GM introduced full-range misfire detection in 1997. This system senses misfire under all positive speed and load conditions up to vehicle "redline." Misfire is not monitored under negative speed and load conditions (deceleration). Misfire detection was only enabled under speed and load conditions to meet FTP standards before 1997.

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Fig. Misfire Monitor Test Graphic

EVAP System Monitor

The EVAP System Monitor is a PCM diagnostic run once per trip that monitors the Evaporative Emissions Control System in order to detect a loss of system integrity or leaks of more than 0.040" in the system.

The GM vehicles included in this section are equipped with an EVAP Purge System or an Enhanced EVAP System (1997-2002 models).

Enhanced EVAP System

The Enhanced EVAP System includes an EVAP canister, fuel tank vapor pressure (FTVP) sensor, purge and vent solenoids, a fuel level sensor, a service port and the fuel cap. The FTVP sensor is used as part of a diagnostic strategy that is based on applying vacuum to the system and then monitoring the amount of vacuum decay over time.

Once the test criteria are met, source vacuum is used to draw a small amount of vacuum on the entire system by mechanically sealing off the designed vent path. After a calibrated amount of vacuum has been achieved, the vacuum source is turned off (system is closed). Leaks are detected by testing the amount of vacuum decay over time.

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Fig. Enhanced EVAP System Graphic

EVAP Purge System

This system includes an EVAP canister (with vapor lines), a purge solenoid (with purge lines), a vacuum switch, fuel tank and cap and related pipes and hoses. The vacuum switch is used to detect when the system is purging. It senses the flow of vacuum from the engine through the purge valve. It is closed when the system is purging.

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Fig. EVAP Purge System Graphic

Fuel System Monitor

The Fuel System Monitor is a PCM diagnostic that continuously monitors the Fuel system to verify its ability to provide compliance with OBD II regulations. This diagnostic must fail if it detects a fault that could cause tailpipe emissions to exceed the FTP standards.

Idle Air Control System Monitor

The PCM controls the air entering into the engine with an Idle Air Control valve. To increase the idle speed, the PCM commands the pintle inside the valve away from the throttle body seat. This action allows more air to bypass through the throttle blade. To decrease the engine speed, the PCM commands the pintle towards the seat. This action reduces the amount of air bypassing the throttle blade.

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Fig. IAC System Graphic

Input Devices And The CCM

Devices that provide input signals are monitored for circuit continuity and out-of-range values including performance checks. Performance checks are defined as checks that indicate a fault when the signal from a sensor does not seem reasonable. An example would be a high TP sensor input at low engine load (e.g., with a low MAP input).

PCM Input Devices that are monitored by the CCM include:

Camshaft and Crankshaft Position Sensors
ECT, IAT, MAF, MAP, TFT and TP Sensors
Knock Sensor and Vehicle Speed Sensor

Long And Short Fuel Trim

The Fuel System Monitor is designed to monitor the averages of Long Term fuel trim (LONGFT) and Short Term Fuel Trim (SHRTFT). If the PCM determines that the fuel trim values reach and stay at their limits for too long a period of time, a fault is indicated and a code is set.

This Monitor compares the average of LONGFT values with SHRTFT values to the rich and lean limits (calibrated fail thresholds) of the test. A Pass is recorded if either value is correct with the system controlling fuel authority. If both values fall into the failure thresholds, a failure condition is exists. In if occurs a code to set and the engine conditions at that time are recorded in Freeze Frame and in the Failure Records.

The Fuel System Monitor also conducts an intrusive test to detect if a rich condition is being caused by excessive vapors from the canister.

Misfire Monitor

The Misfire Monitor is a PCM diagnostic required by OBD regulations that continuously monitors the engine for misfires under all engine positive load and speed conditions. The test operates on the principle that crankshaft rotational velocity fluctuates as each engine cylinder contributes its power. When a misfire occurs, the crankshaft speed will slow down temporarily.

The PCM monitors crankshaft rotational velocity using a crankshaft position (CKP) sensor. The CMP sensor is for cylinder identification.

Catalyst Damaging Diagnostic

This diagnostic operates when the level of misfire is sufficient to cause catalyst damage under the current operating conditions. It is designed to detect a misfire within 200-1000 crankshaft revolutions. If a Type 1-Trip code is set (or a 2-Trip code with misfire relief), the PCM will command the MIL to flash. If a catalyst-damaging misfire is no longer present, the MIL will remain on steady instead of flashing.

Emissions - Threatening Diagnostic

The Misfire test is designed to detect levels of misfire sufficient to result in emissions levels that exceed 1.5 times the FTP standard. It is designed to detect misfire within 1000-4000 crankshaft revolutions. The PCM arms the Misfire trouble code (2-Trip code) on the first trip. If the same misfire is detected on two consecutive trips, the PCM will store a History Code and illuminate the MIL. The PCM will also set this type of code and illuminate the MIL if it detects a misfire condition on non-consecutive trip if the misfire occurs under the same operating conditions (within 375 rpm of engine speed and 20% of engine load under similar engine coolant temperature) in the last 80 trips.

Misfire Definition

Misfire is defined as lack of combustion in a cylinder due to weak compression, lack of adequate ignition spark, poor fuel metering, or any other engine mechanical, fuel or ignition system fault.

Monitor Tests

An OBD II Monitor is a PCM controlled (diagnostic) test run on one or more of the Emission Control systems to determine if a component or system is operating properly. As discussed, some tests either run once per trip or run continuously. The OBD II Monitor tests include:

Catalyst & Fuel System Monitors
EGR & EVAP Monitors
Misfire Monitor
Oxygen Sensor & Oxygen Sensor Heater Monitors
Secondary AIR Monitor

Non-Monitored Systems & Components

The PCM and CCM cannot monitor the Base Engine systems and components for faults or conditions that might cause a driveability problem. This situation can cause some confusing diagnostic situations when a Base Engine problem is present in this system.

Engine Compression

The PCM cannot detect uneven, low, or high engine cylinder compression. However, a fault in one of these areas could cause the Oxygen Sensor or Misfire Monitor to fail during testing.

Exhaust System

The PCM cannot detect a restriction or leak in the Exhaust system. However, a fault in one of these areas could cause the EGR System, Fuel System, HO2S or Misfire Monitor to fail during testing.

Fuel Injector Mechanical Fault

The PCM cannot detect if a fuel injector is restricted, stuck open or closed. However, a fault in one of these areas could result in a rich or lean condition and cause the Fuel System or HO2S Monitor to fail.

Fuel Pressure

The fuel pressure regulator controls fuel system pressure. The PCM cannot detect a restricted fuel pump inlet filter, a dirty in-line fuel filter, or a pinched fuel supply or return line. If the fuel pressure is too high or low, a fuel pressure code would not set, but a misfire or oxygen sensor code might set due to a lean or rich A/F condition.

Ignition System Secondary

The PCM cannot detect a faulty ignition coil, fouled or worn out spark plugs, ignition wires that are cross firing, or an open spark plug wire. However, the Misfire Monitor would detect these faults during testing.

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Fig. Engine Analyzer Testing the Ignition System

Output Devices & The CCM

The PCM uses an Output Device Monitor to "watch" the voltage level change on a controlled device as it is switched from "on" and "off". The eyeballs in the Graphic below represent the point at which the PCM monitors the output command signal for a change in voltage.

PCM (controlled) Output Devices monitored by the CCM include:

Air Conditioning, Cooling Fan and Fuel Pump Relays EVAP Purge and EVAP Vent Control Solenoids
Cruise Control Vacuum and Vent Solenoids
Electronic Transaxle Controls
Idle Air Control Motor and Malfunction Indicator Lamp

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Fig. Output Device Monitor Graphic


Oxygen Sensor Heater Monitor

OBD II regulations require the Oxygen Sensor Heater Monitor to test the operation of the front and rear oxygen sensor heaters. This type of diagnostic is conducted right after a cold engine startup occurs.

Fuel Control HO2S Operation

The main function of the front oxygen sensor (O2S or HO2S) is to provide the PCM with exhaust stream information to allow proper fuel control. Once it reaches operating temperature, the sensor generates a voltage that is inversely proportional to the amount of oxygen in the exhaust gases. Once the system is in closed loop, the PCM uses the signal from the oxygen sensor to adjust the injector pulsewidth to maintain an A/F ratio that controls tailpipe emissions and driveability.

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Fig. HO2S Response Time Graphic

HO2S Heater Test

The HO2S Heater Test measures the time required for the HO2S to become active after a cold engine startup. The time required for the oxygen sensor to become active is compared to a calibrated fault threshold in the PCM to determine the capability of the HO2S heater.

If the heater circuit has deteriorated or failed, the amount of time required for the HO2S to become active will increase. If the engine is started warm, or if a heater failure occurs during normal operation, other HO2S tests are used to detect a fault in the heater or its circuit.

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Fig. Heated Oxygen Sensor Graphic

Oxygen Sensor Monitor

OBD II regulations require that the Oxygen Sensor Monitor test the front and rear oxygen sensors for faults or deterioration that could cause tailpipe emissions to exceed 1.5 times the FTP standard. The Oxygen Sensor Monitor continuously tests the voltage output and response rate of the front and rear oxygen sensor (see the Graphic).

Rough Road Detection

Driving over rough roads can cause engine speed variations that are similar to an engine Misfire condition and may cause false detection of a misfire. OBD II regulations allow the misfire detection test to be disabled for a short time while driving on rough road conditions.

A rough road can cause torque to be applied to the drive wheels and drive train. This torque can temporarily and intermittently decrease engine speed, and thereby cause the Misfire Monitor to incorrectly detect a misfire condition.

Misfire Counters

Whenever a misfire is detected, the Misfire Monitor counts the misfire and notes the crankshaft position at the time that the misfire occurs. These misfire counters consist of a file on each engine cylinder.

A Current and History misfire counter is maintained for each cylinder. Misfire Current Counters (Misfire Current Cyl 1-8) indicate the number of firing events out of the last 200 crankshaft revolutions that were misfires. Misfire Current Counters display real time data without a misfire code stored. Misfire History Counters (Misfire History Cyl 1-8) indicate the total number of cylinder firing events that were misfires. Misfire History counters display zero (0) until the Misfire Monitor has detected a failure and DTC P0300 is set.

Once the DTC P0300 sets, the Misfire History counters are updated every 200 revolutions of the crankshaft. If the Misfire Monitor reports a failure, the Diagnostic Executive reviews all of the Misfire counters before reporting a trouble code (to report the most current data).

Secondary Air Monitor

Some GM vehicles are equipped with a Secondary Air Injection (AIR) system. OBD II regulations require that the presence of secondary airflow in the exhaust stream be monitored along with the functional monitoring of the AIR pump and its related switching valves.

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Fig. Secondary AIR System Graphic

Secondary Air Monitor Active Test

If a Passive Test fails or is inconclusive, the Active Test is started. The AIR pump is turned on during closed loop operation, and the Monitor will indicate Pass or Fail based on the HO2S-11 and SHRTFT values. A low HO2S-11 signal and increase in SHRTFT values indicates the AIR system is operating as designed.

Secondary Air Monitor Passive Test

When the Secondary AIR system is activated, excess air flows into the exhaust causing the HO2S-11 signal to drop. Then the PCM monitors the HO2S-11 signal and/or SHRTFT value for a response.

A Passive Test monitors the HO2S-11 signal after startup and also prior to closed loop operation. The AIR pump is normally enabled at this point to clean-up exhaust emissions. The HO2S-11 signal should be approximated 0-200mv if the AIR pump is delivering additional air to the exhaust during this period. The AIR Monitor will indicate a Pass if HO2S-11 indicates lean prior to closed loop operation. The Monitor also looks for the HO2S-11 to toggle as the AIR pump is disabled.

Systems & Terminology

Cylinder Bank Identification

Engine sensors are identified for each engine cylinder bank by SAE regulations as explained below.

Bank - A specific group of engine cylinders that share a common control sensor (e.g., Bank 1 identifies the location of Cyl 1 while Bank 2 identifies the cylinders on the opposite bank).

An example of the cylinder bank configuration for a Chevrolet Lumina with FWD and a 3.4L V6 (VIN X) transverse mounted engine is shown in the Graphic to the right.

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Fig. Cylinder Bank Identification Graphic

Data Link Connector

OBD II Systems use a standardized test connector called the Data Link Connector (DLC). It is located beneath the instrument panel somewhere between the left end of the instrument panel and 12 inches (300 mm) past the vehicle centerline.

The DLC is located out of the sight of vehicle passengers, but should be easily viewable by a technician from a kneeling position outside the vehicle. The DLC is rectangular in design, capable of accommodating up to 16 terminals and has keying features to allow for easy connection. The DCL and Scan Tool connector have latching features that ensure the Scan Tool connector will remain mated when properly connected. Some common uses of the Scan Tool are:

To identify and clear any stored diagnostic trouble codes
To read the serial data stream information (e.g., PID data)
To perform Enhanced Diagnostic Tests (bi-directional Scan Tool)

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Fig. Data Link Connector Graphic


Diagnostic Trouble Code Display

The Scan Tool can display up to five DTC options of enhanced code data on GM Vehicles They are DTC Info, Specific DTC, Freeze Frame, Fail Records (not used on all applications) and Clear Info.

Conditions: To Clear Diagnostic Trouble Codes

Here are 3 methods for clearing codes from the PCM memory:

Use the Scan Tool (also clears Freeze Frame & Failure Records)
If battery power to the PCM is removed (battery cable or PCM fuse), all current data (DTC, Freeze Frame, Fail Records, Statistical Filters and I/M Readiness Flags) will be cleared
If the fault that caused a DTC to set has been corrected, the PCM will begin to count warmup cycles. Once it has counted 40 warmup cycles with no further faults detected, the DTC is cleared

DTC Info Mode

This mode is used to search for a specific type of stored DTC information. There are seven different selections. The repair charts may instruct the technician to test for a DTC in a particular manner.

MIL Request
Last Test Fail
Test Fail Since Codes Cleared
Not Run Since Codes Cleared
Fail This Ignition
DTC Status

Specific DTC Mode

In this Mode, the PCM checks the status of individual diagnostic tests by their DTC number. This selection can be accessed if a DTC has passed, failed or a combination of both. Some of the individual DTC tests that are available on the Scan Tool are shown below:

Failed Last Test
Failed Since Clear
Failed This Ignition
History DTC
MIL Requested
Not Run Since Clear
Not Run This Ignition
Test Ran and Passed

Diagnostic Trouble Codes

Each OBD II Diagnostic Trouble Code (DTC) is directly related to a particular Monitor Test. The Diagnostic Management System sets codes based on the failure of the tests during a trip (or trips). Certain tests must fail during two consecutive trips before the DTC is set.

DLC Pin Assignment Table

An example of the GM DLC Pin Assignments is shown below.

CavityPin AssignmentCavityPin Assignment
1Secondary UART 8192 Baud9Primary UART
2Class II or J1850 Bus + L-Line10J1850 Bus- Line (2-wire)
3Ride Control Diagnostic11EVO or MSVA Steering
4Chassis Ground12ABS or CCM Enable
5Signal Ground13SIR Diagnostic Enable
6PCM/VCM Diagnostic Enable14E & C Bus
7ISO 9141 (K-Line) Bus15ISO 9141 (L-Line) Bus
8Keyless Entry or MRD Enable16Fused Battery Power
Drive Cycle

General Motors OBD II systems implement the usual Strategy Based Diagnostic procedures built into the Powertrain Control Module (PCM) and Transmission Control Module (TCM). The first step in diagnosis of a problem on an OBD II system is to identify it as either a trouble code fault (Code Fault) or a driveability symptom (No Code Fault). The OBD II Drive Cycle is used to verify any repair to the system.

OBD II Drive Cycle Procedure

The main intention of the OBD II Drive Cycle is to run the OBD II Main Monitors in order to determine the status of the Inspection & Maintenance (I/M) Readiness Tests. A cold engine startup (e.g., ambient air temperature of 40-100ºF) is a necessary step in preparation to run a complete OBD II Drive Cycle. In most cases the engine coolant temperature must be below 122ºF.

OBD II Drive Pattern

The drive pattern shown below can be used to help solve:

Trouble Code Faults - Refer to the Code List (in this section) or look in electronic media or repair manuals for a code repair chart.

Driveability Symptoms & Intermittent Faults - Refer to the special repair instructions under No Code Faults in other repair manuals.

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Fig. GM OBD II Drive Cycle Graphic

Strategy Based Diagnostics

Strategy Based Diagnostics can be used to repair all Electrical and Electronic systems on GM vehicles. The diagnostic approach in this methodology can also be used to solve problems in an OBD II system.

Enable Criteria

The term enable criteria is used to describe the exact Conditions: necessary for a diagnostic test to run. Each Monitor has a specific set of Conditions: that must be met before the diagnostic test is run. It may include, but is not limited to the following items:

Air Conditioning (A/C) on
BARO, ECT, IAT, MAP, TP and Vehicle Speed Sensors
EVAP Canister Purge Enabled or Disabled
Engine Speed (RPM) and Engine Load
Short Term and Long Term Fuel Trim

Failure Records

Failure Records are records in the PCM that contain the engine operating conditions present when a trouble code is stored and the MIL is illuminated. On GM vehicles, the PCM can store multiple records (i.e., the Aurora 4.0L V8 engine can store up to three records while other vehicles can store up to five records) and can update the records at any time. As each record is updated, the first record is dropped and the new failure event is recorded. These records are an enhancement of the OBD II Freeze Frame capture feature, but they can store data for any fault in memory (not just data for a MIL fault)

GM vehicles can store up to 5 Failure Records on some models. Each record is for a different code. It is also possible that there will not be Failure Records for every code if multiple codes are set. The four types of codes and their related characteristics are shown below:

Failure records store data about the operating conditions when the code was stored and the MIL was illuminated. The PCM can store multiple records and can also update these records at any time. Some vehicles can store up to 5 failure records. However, the Aurora with the Northstar 4.0L engine will only store up to 3 failure records.

The current engine operating conditions are recorded in the Failure Records buffer each time a test fails. As each record is updated, the first record is dropped and the new failure event is recorded.

The operating conditions for a diagnostic test that failed may include one or more of the following engine operating parameters:

Air Fuel Ratio
Airflow Rate
Barometric Pressure
Engine Load
Engine Coolant Temperature
Engine Speed
Fuel Trim
Injector Base Pulsewidth
Manifold Absolute Pressure
Open or Closed Loop Status
Throttle Position Angle
Vehicle Speed

Flash Eeprom

GM vehicles with OBD II systems (including 1991-95 Saturn models) use a flash erasable programmable read only memory device to make running changes.

In most cases, a computer is used to download the latest changes (listed by VIN code) into a PC or the Scan Tool. The service bay PC or Scan Tool is then connected to the vehicle so that it can verify the current PROM calibration to determine if the PROM needs updating.

Freeze Frame Data

Freeze Frame is an element of the Diagnostic Executive that stores engine operating conditions at the moment an emission-related fault is stored in memory (when the MIL is commanded on). This data can be used to help identify the cause of an emissions-related fault.

Regulations related to the OBD II System require that certain engine-operating conditions be captured and stored whenever the MIL is illuminated. The data captured is called Freeze Frame data. This data can be thought of as a single record of a certain set of operating conditions. Whenever the MIL is turned on, the corresponding record of operating conditions is recorded to the Freeze Frame buffer.

The Freeze Frame data can only be overwritten by data associated with a Fuel Trim or Misfire fault because data from these faults takes priority over data associated with any other type of fault. The Freeze Frame data will not be erased unless the associated History DTC is cleared.

Fuel Trim Diagnostics

In order to meet OBD II regulations, fuel trim information is displayed on a Scan Tool in percentages. This is different from the way fuel trim has been traditionally displayed on a Scan Tool. Short term and long term fuel trim functions within OBD II are similar to past usage, only their measurement units will differ. The fault detection logic and MIL operation is the same as that described for Misfire Diagnosis.

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Fig. Fuel Trim Conversion Graphic

Heated Oxygen Sensor

The Heated Oxygen Sensor (HO2S) detects the presence of oxygen in the exhaust and produces a variable voltage according to the amount of oxygen detected. The HO2S outputs a voltage from 0-1v. A value less than 0.4v indicates lean A/F ratio and a value over 0.6v indicates a rich A/F ratio.


In the V6 and V8 examples in the Graphic above, HO2S-11 refers to the upstream oxygen sensor and HO2S-12 refers to the downstream oxygen sensor. The downstream oxygen sensor or third oxygen is referred to as HO2S-13. The upstream HO2S-11 (or HO2S-12 on the V6 and V8 engines as noted) signal is used with the Oxygen Sensor Monitor test function. The HO2S-12 (or HO2S-13 as noted) signal is used with the Catalyst Monitor test function.

HO2S-11, Hos-12, HO2S-13 Locations - Example 1

Throughout the manual, there are references to Heated Oxygen Sensors identified with acronyms HO2S-11, HO2S-12 and HO2S-13. In addition, the sensors are identified as either cylinder Bank 1 or cylinder Bank 2. It should be understood that Bank 1 always contains engine cylinder number 1 (Cyl 1).

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Fig. HO2S Location Graphic

Heated Oxygen Sensor (Continued)

In both of the pictures in Graphic above, HO2S-11 refers to the upstream oxygen sensor while HO2S-12 refers to the downstream oxygen sensor.

In these examples, the upstream HO2S-11 signal is used with the Oxygen Sensor Monitor test function. The downstream HO2S-12 signal is used with the Catalyst Monitor test function. HO2S location information is very important when attempting to identify the correct oxygen sensor as it relates to a trouble code repair chart.

HO2S-11, Hos2 Locations - Example 2

Throughout the manual, there are references to Heated Oxygen Sensors identified with acronyms HO2S-11, HO2S-12 and HO2S-13. In addition, the sensors are identified as either cylinder Bank 1 or cylinder Bank 2. It should be understood that Bank 1 always contains engine cylinder number 1 (Cyl 1).

Click image to see an enlarged view

Fig. HO2S Location Graphic

I/M Readiness Status

The Scan Tool can identify the Flags or I/M Readiness Status. A flag ON for a system means that the test has been run. A flag OFF for a system means that the test has not been run. If a vehicle comes in with a problem, the technician should first look at the "Flags" screen on the Scan Tool to see if all flags were set to ON. If the EGR flag is OFF, there is a possibility that the EGR system has a fault or that the EGR system tests have not have been run (the EGR system may be okay or it may have a problem). The technician needs to drive the vehicle under the trip Conditions: and get the flag set to ON before proceeding with testing the EGR system. It should be noted that OBD II trips are different for vehicles with different body codes and engines.

If power to the PCM is removed (by removing a fuse or disconnecting the battery) the Inspection & Maintenance (I/M) Flags will be reset to "off". In effect, the vehicle must be driven under specific Conditions: until all flags are set to "on".

If the power is removed, a Scan Tool can be used to do a "quick relearn step" which resets the Fuel Trim and Idle Speed to the default settings. These steps are part of the MISC, FUNCTIONAL or SPECIAL menus found on many Aftermarket Scan Tools.

Malfunction Indicator Lamp

The Malfunction Indicator Lamp (MIL) looks similar to the lamp used on earlier vehicles (i.e., the Check Engine or "Service Engine Soon lamp). However, on OBD II systems, the MIL is activated under a strict set of guidelines that dictate that the lamp must be turned on when the PCM detects an emissions related fault that could impact the vehicle tailpipe emissions or Evaporative Loss system.

The Malfunction Indicator Lamp (MIL) lamp is mounted in the instrument panel. It provides several functions as described below:

To inform the driver that a fault affecting vehicle emission levels has occurred and to bring the vehicle in for service immediately.
To check the bulb and related circuit, the MIL will is turned on with the key in the "on" position with the engine off (KOEO). Once the engine is started, the MIL should go out.
If the MIL remains on with the engine running, or if a fault is suspected due to an emissions related problem, an OBD System Check of the PCM diagnostics should be performed. The tests contained in this diagnostic procedure will help find any faults that might not be detected using other diagnostic routines.

Click image to see an enlarged view

Fig. MIL Circuit Graphic


Malfunction Indicator Lamp (Continued)

Once the MIL is on, the Diagnostic Executive will turn the MIL off after three consecutive trips when test passed is recorded for the trouble code that originally caused the MIL to be activated.

If this situation occurs while the MIL is off the DTC that set when the emission-related fault occurred will be stored by the PCM in the Freeze Frame and Failure Records. The DTC will remain in memory until 40 warmup cycles (without a new fault) have been completed.

If the MIL was set due to either a Fuel system or Misfire related fault, there are other requirements to meet before the code can be cleared. Once all the requirements for these types of faults are met, the onboard diagnostics can validate that the emissions fault that caused the MIL to be activated has been corrected.

The additional requirements for a Fuel system or Misfire fault are:

Diagnostic tests that are passed must occur within 375 rpm of the engine speed data stored at the time that the last test failed
The engine must be at 10% of the engine load that was stored at the time the last test failed
Engine operating conditions must be similar to the conditions present (warmed up or warming up) when the last test failed

Intermittent MIL -On- Conditions:

If the PCM detects a fault and then the fault goes away, the MIL will remain on until after three trips are completed without the same fault reoccurring. This type of MIL "on" condition could appear to be an intermittent fault, but most OBD II faults are not intermittent in nature.

An example of an intermittent MIL condition is described next. If the customer were to leave the fuel filler cap loose or off, and the vehicle was driven under the correct code conditions (meeting all enable criteria for an EVAP large leak trouble code), the PCM would turn the MIL on the second time it ran the EVAP Monitor and failed the test. The MIL would remain on until the vehicle was refueled and the customer tightened the fuel cap properly or noticed it was off. Once the vehicle was driven under the correct EVAP large leak code conditions, the EVAP Monitor would run the test. If the EVAP test passed for three consecutive trips, the PCM would turn off the MIL (this is true for some vehicles depending upon the model year).

However, in this case, the related trouble code would remain in memory until 40 OBD II warmup cycles occur (80 OBD II warmup cycles for Fuel system and Misfire faults) without the fault reoccurring. If the vehicle was brought in for service and a PCM Reset step was done, the MIL would then go out and the codes would clear.

MIL On/Off Guidelines

If an emission-related fault is detected, the Diagnostic Executive will activate the MIL and allow it to remain "on" until the system or component passes the same test for three consecutive trips without the fault reoccurring.

MIL On Or Flashing - Fuel System Or Misfire Conditions:

If a Fuel system problem or misfire condition is present that could damage the catalyst, the MIL will flash once per second. The MIL will continue to flash until the vehicle is outside of the speed and load conditions that could cause possible catalyst damage. It will stop flashing and remain on once these conditions: are no longer present.

Misfire Diagnostics

The Diagnostic Executive has the capability of alerting the driver of potentially damaging levels of engine misfire that could damage the catalyst. If this type of condition is detected, the MIL is commanded to flash on/off once per second during the actual misfire condition.

The Misfire Diagnostics represents a special type of trouble code diagnostics. Each time a misfire is detected, the engine load, engine speed and coolant temperature at that moment are recorded and the last reported set of Conditions: are stored in Freeze Frame when the key is turned off. During the next few key on/off cycles, the stored Conditions: are used as a reference for similar conditions.

If an emissions threatening misfire occurs for two consecutive trips, the PCM treats the fault as a normal Type B trouble code (it turns on the MIL and stores a code). However if a misfire is detected on two non-consecutive trips, the stored Conditions: are compared with the current conditions. If a misfire occurs on a second non-consecutive trip under the similar Conditions: listed below, the MIL is illuminated:

The engine load conditions are within 10% of the previous failure
Engine speed is within 375 rpm of the previous failure
Engine temperature is in the same range as the previous failure

Monitor Software

The Diagnostic Executive contains software designed to allow the PCM to organize and prioritize the Main Monitor tests and procedures, and to record and display test results and diagnostic trouble codes. The functions controlled by this software include:

To control the diagnostic system so that the vehicle will continue to operate in a normal manner during testing.
To ensure that all of the OBD II Monitors run during the first two sample periods of the Federal Test Procedure.
To ensure that all OBD II Monitors and their related tests are sequenced so that required inputs (enable criteria) for a particular Monitor are present prior to running that particular Monitor.
To sequence the running of the Monitors to eliminate the possibility of different Monitor tests interfering with each other or upsetting normal vehicle operation.
To provide a Scan Tool interface by coordinating the operation of special tests or data requests.

Similar Conditions

For Fuel System and Misfire codes, the engine operating conditions must be similar to conditions present when the fault was first detected to clear a code. In effect, the engine load must be within 10%, engine speed within 375 rpm and the engine temperature similar (cold or warm) before the Fuel System or Misfire Monitor will retest for a code.

Standard Corporate Protocol

On vehicles equipped with OBD II, a Standard Corporate Protocol (SCP) communication language is used to exchange bi-directional messages between stand-alone modules and devices. With this type of system, two or more messages can be sent over one circuit.

Trip Definition

An OBD II trip is official when all the enable criteria for a given test (Monitor) are met. Because enable criteria vary from one Monitor to another, the definition of a trip varies as well. The trip requirements (criteria) can include seemingly unrelated items such as driving style, the length of the trip, and ambient temperature. A minimum requirement for a trip includes one key cycle, and in most cases, the engine must run for a period of time before the test is enabled.

Vehicle tests vary in length - some are performed only once per trip and some are performed continuously. The Catalyst, EGR, EVAP and Oxygen Sensor tests are performed once per trip. The Component Monitor, Fuel and Misfire tests are performed continuously. An OBD II trip is defined as a "key on-drive-the-vehicle-key-off" cycle in which the vehicle is operated in a manner that satisfies the criteria for a test.

Type 'A' Codes

Type 'A' Codes are emissions-related

Request the MIL "on" the first trip a fault is detected
Store a History Code on first trip a fault is detected
Store Freeze Frame data the first trip a fault is detected (if empty)
Store a Failure Record
Update the Failure Record each time a Monitor Test fails

Type 'B' Codes

Type 'B' Codes are emissions-related

Are "armed" or pending after one trip when a fault is detected
Are "disarmed" after 1-Trip if the second consecutive trip passes
Request the MIL "on" the 2nd consecutive trip if the fault occurs
Set a History Code on the 2nd consecutive trip if the fault occurs
Store Freeze Frame Data the 1st trip that a fault is detected
Store a Failure Record the first trip a fault is detected
Update a Failure Record the first time the test fails each key cycle

Type 'C' Codes (Changed To Type C1 In Mid-1997)

Type 'C' Codes are non-emissions related

Request the SES (lamp) or DIC the first time the fault is detected
Store a History Code the first time the fault is detected
Do not store a Freeze Frame Record
Store a Failure Record the first time the test fails each key
Update a Failure Record the first time a test fails each key cycle

Type 'D' Codes (Changed To Type C0 In Mid-1997)

Type 'D' Codes are non-emission related

Do not request the PCM or other controller to turn on a lamp
Store a History Code on the first trip that a fault is detected
Do not store a Freeze Frame Record
Store a Failure Record if a non-emission related test fails
Update the Failure Record each time a non-emissions test fails

Warm-up Cycle

Once the MIL is off, a stored trouble code will remain in memory until 40 warmup cycles are completed without the fault occurring.

A warmup cycle is defined as a trip that includes a change in engine temperature of at least 40ºF and where it reaches 160ºF.

Click image to see an enlarged view

Fig. OBD II Warmup Graphic

UART Serial Data

There are two methods of data transmission used on OBD II systems. One method is the Universally Asynchronous Receiving/Transmitting (UART) protocol. UART is an interface device that allows a controller to send and receive serial data. Serial data refers to information that is transferred in a linear fashion (over a single line) one bit at a time. A data bus describes the electronic pathway through which serial data travels. UART receives serial data, converts the data to parallel format, and then places it on the data bus for use by the vehicle computer. UART can also convert data in parallel format to serial format, and then transmit the converted data to the Scan Tool.