GM Service Manual Online
For 1990-2009 cars only

Emissions Related DTCs

Action Taken When the DTC Sets - Type A

The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.

Action Taken When the DTC Sets - Type B

The control module illuminates the MIL on the second or third consecutive ignition cycle that the diagnostic runs and fails.

Action Taken When the DTC Sets - Type B1

    â€¢ The following applies to misfire DTCs:
       - If the control module detects a low level or an emission level misfire condition during 2 consecutive trips, the control module illuminates the MIL.
       - If the control module detects a high level or catalyst damaging misfire, the control module flashes the MIL at a rate of once per second.
       - If the control module detects a misfire during 2 non-consecutive trips, the stored conditions are compared with the current conditions. The control module illuminates the MIL when the following conditions occur:
   â€¢ The engine load is within 10 percent of the previous test that failed.
   â€¢ The engine speed is within 375 RPM of the previous test that failed.
   â€¢ The engine coolant temperature is in the same range of the previous test that failed.
    â€¢ The following applies to fuel trim DTCs:
       - If the control module detects a fuel trim condition during 2 consecutive trips, the control module illuminates the MIL.
       - If the control module detects a fuel trim condition during 2 non-consecutive trips, the stored conditions are compared with the current conditions. The control module illuminates the MIL when the following conditions occur:
   â€¢ The engine load is within 10 percent of the previous test that failed.
   â€¢ The engine speed is within 375 RPM of the previous test that failed.
   â€¢ The engine coolant temperature is in the same range of the previous test that failed.

Conditions for Clearing the MIL/DTC - Type A or Type B

    â€¢ The control module turns OFF the MIL after 3 or 4 consecutive ignition cycles, when the diagnostic runs and passes.
    â€¢ An active DTC clears when the diagnostic runs and passes.
    â€¢ A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
    â€¢ Use a scan tool in order to clear the MIL and the DTC.

Non-Emissions Related DTCs

Action Taken When the DTC Sets - Type C

    â€¢ The control module stores the DTC information into memory when the diagnostic runs and fails.
    â€¢ The MIL will not illuminate.
    â€¢ The driver information center, if equipped, may display a message.

Conditions for Clearing the DTC - Type C

    â€¢ A last test failed, or active DTC, clears when the diagnostic runs and passes.
    â€¢ Use a scan tool in order to clear the DTC.

Type X

No actions occur with a type X DTC. These DTCs are coded into the control modules software, but the DTC will not run because one of the following reasons:

    â€¢ The associated hardware is not installed with the vehicles emission package.
    â€¢ The diagnostic is not required for the vehicles emission package.