GM Service Manual Online
For 1990-2009 cars only

Class 2 Communications

The United States federal regulations require that all automobile manufacturers utilize a common serial data communications system. This system is called the Class 2 communications system. The Class 2 communications system allows multiple signals, or messages to be carried on a single serial data circuit.

The Diagnostic Executive

The Diagnostic Executive is the name given to a portion of the of the control module programming stored in the EEPROM. The functions of the Diagnostic Executive are as follows:

    â€¢ To monitor on-board diagnostic test status information
    â€¢ To record on-board diagnostic test status information
    â€¢ To record pending DTCs
    â€¢ To record current DTCs
    â€¢ To turn the MIL ON
    â€¢ To store and erase Freeze Frame/Failure Records data
    â€¢ To turn the MIL OFF
    â€¢ To record History DTCs

On-Board Diagnostic Test

When the test enabling conditions for a particular test are met, the test will continue to run until one of the following occur:

    â€¢ The conditions for enabling that test are no longer present.
    â€¢ The test is completed and passed. In this instance the Diagnostic Executive will store the following information:
       - The test has run since the last ignition cycle.
       - The test has passed during the current ignition cycle.
       - A malfunction is not present at this time.
    â€¢ The test is completed and failed. In this instance the Diagnostic Executive will store the following information:
       - The test has run since the last ignition cycle.
       - The malfunction is present at this time.
       - Certain engine data values present at the time of the malfunction.

Ignition Cycle

An ignition cycle consist of the following steps:

  1. Start the engine.
  2. Some diagnostic tests require a warm-up cycle in order to run.

  3. Operate the vehicle within the conditions for running the particular diagnostic test. The conditions required to run each test vary and may require the following in order to run:
  4. • A particular style of driving
    • A certain amount of time
    • A certain outside air temperature
  5. Turn the ignition OFF.

Some diagnostic tests will only run one time during an ignition cycle. Some diagnostic test run continuously. Some of the diagnostic test may start to run, and stop . Some diagnostic tests may not run within the ignition cycle.

Warm-Up Cycle

A warm-up cycle consist of the following:

    â€¢ An engine start-up.
    â€¢ An increase in engine coolant temperature (ECT) of more than 22°C (40°F) above start-up engine coolant temperature.
    â€¢ A minimum ECT of 71°C (160°F).

Certain diagnostic test will not run if these conditions are not met during an ignition cycle.

Freeze Frame/Failure Records

If the control module turns the MIL ON because a DTC is set, the following information is stored in the scan tool and displayed in the Freeze Frame/Failure Records data list.

    â€¢ The DTC that caused the MIL to be turned ON.
    â€¢ The engine speed (RPM) at the time the DTC set.
    â€¢ The Engine Coolant Temperature (ECT) at the time the DTC set.
    â€¢ The Manifold Absolute Pressure (MAP) at the time the DTC set.
    â€¢ The Short Term (ST) Fuel Trim for Bank 1 at the time the DTC set.
    â€¢ The Long Term (LT) Fuel Trim for Bank 1 at the time the DTC set.
    â€¢ The Short Term (ST) Fuel Trim for Bank 2 at the time the DTC set.
    â€¢ The Long Term (LT) Fuel Trim for Bank 2 at the time the DTC set.
    â€¢ The Loop Status at the time the DTC set.
    â€¢ The Engine Load at the time the DTC set.
    â€¢ The Mass Air Flow (MAF) at the time the DTC set.
    â€¢ The Vehicle Speed at the time the DTC set.
    â€¢ The Mileage Since First Failure
    â€¢ The Mileage Since MIL Request

This information will remain stored in the control module until the following occurs:

    â€¢ A misfire DTC is set, and that information is stored.
    â€¢ A fuel trim DTC is set, and that information is stored.
    â€¢ The DTC information is cleared.

Reading Diagnostic Trouble Codes (DTC)s

Use a scan tool in order to read Diagnostic Trouble Codes (DTC)s. Failure to follow this procedure will result in unnecessary repairs.

Clearing Diagnostic Trouble Codes (DTC)s

Diagnostic Trouble Codes (DTC)s will be cleared by one of the following ways :

Notice: Turn off the ignition key when disconnecting or reconnecting battery power in order to prevent system damage.

Important: Disconnecting the negative battery will result in the loss of other on-board memory data, such as preset radio tuning.

    â€¢ Use a scan tool by following the instructions supplied by the manufacturer.
    â€¢ When the vehicle has completed 40 warm-up cycles and not detected a failure.

DTC Types

There are 3 types of DTCs. They are as follows:

    â€¢ The Type A DTC.
        This DTC is an emissions related DTC. The first time a type A DTC diagnostic runs and fails the MIL is turned ON.
    â€¢ The Type B DTC.
        This DTC is an emissions related DTC. The MIL is turned ON when a diagnostic test runs and fails two consecutive ignition cycles.
    â€¢ The Type C DTC.
        This DTC is not an emissions related DTC. The MIL is not turned ON when this type of diagnostic test runs and fails.

Malfunction Indicator Lamp (MIL)

The Malfunction Indicator Lamp (MIL) is located at the instrument panel. The MIL (Service Engine Soon) has the following functions:

    â€¢ Bulb check. The MIL is turned ON when the ignition switch is in the Run or ON position.
    â€¢ System check. When the engine is started, the MIL is turned OFF if no DTCs are set.
    â€¢ The MIL informs the driver that a malfunction that affects the emission control system of the vehicle has occurred. The driver is to take the vehicle in for service as soon as possible.
    â€¢ In order to alert the driver that a catalytic converter damaging engine misfire condition is currently present, the MIL is turned ON and OFF once per second.

Important: The MIL is turned ON for the above conditions only. If the engine stops running with the ignition switch in the Run or the ON position the MIL is not turned ON.

Perform the Powertrain On-Board Diagnostic (OBD) System Check if the following conditions exist:

    â€¢ The engine is running, and the MIL is ON.
    â€¢  A malfunction is suspected due to a driveability symptom.

Intermittent MIL (Service Engine Soon)

If a malfunction is intermittent , the MIL (Service Engine Soon) may turn ON and then turn OFF after 3 ignition cycles. However, the VCM will store the corresponding DTC and certain engine operating data present at the time the malfunction is detected.

Data Link Connector (DLC)

The Data Link Connector (DLC) is located under the instrument panel. The DLC is the provision for connecting the scan tool to the serial data communications circuit in order to perform the following:

    â€¢ Perform the Powertrain On-Board System Check.
    â€¢ Identify the DTCs stored by the VCM.
    â€¢ View the scan data
    â€¢ Monitor the I/M System Status
    â€¢ Perform the system tests
    â€¢ Perform the diagnostic tables
    â€¢ Clear the DTCs from the VCM memory
    â€¢ Verify the repair of the vehicle

Engine Controls Diagnostic Information

The Engine Controls Diagnostic Information and Procedures consist of diagnostic tables and functional checks that are developed in order to locate a faulty circuit, component, or system through a logical process of elimination. The diagnostic tables are prepared with assumption that the vehicle functioned correctly at the time of assembly and that there are not multiple malfunctions present.

Verifying Vehicle Repair

Following a repair, the technician is to verify that the malfunction is repaired. Verify the repair by performing the following steps:

  1. Review and record the Freeze Frame/Failure Records data for the DTC which was diagnosed. The control module will store Freeze Frame/Failure Records data for type A or a type B DTC only, and only then if the MIL has illuminated.
  2. Clear the DTCs.
  3. Operate the vehicle within the conditions recorded in the Freeze Frame/Failure Records data.
  4. Monitor the DTC Status data for the specific DTC which has been diagnosed until the diagnostic test associated with that DTC runs.