GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The powertrain control module (PCM) detects engine misfire by detecting variations in crankshaft deceleration between firing strokes. For accurate detection of engine misfire, the PCM must distinguish between crankshaft deceleration caused by actual misfire and deceleration caused by rough road conditions. The antilock brake system (ABS) can detect if the vehicle is on a rough road based on wheel acceleration or deceleration data supplied by the wheel speed sensors. If the ABS detects rough road above a predetermined threshold, this information is sent to the PCM via serial data. The PCM can then take the rough road into account when calculating misfire. Even if the ABS is malfunctioning and cannot detect rough roads, the misfire diagnostic will continue to run. However, if a misfire diagnostic trouble code (DTC) sets, this additional DTC also sets indicating that rough road data was not available during the misfire calculation due to a serial data malfunction.

Conditions for Running the DTC

    • The vehicle speed is above 16 km/h (10 mph).
    • The engine speed is below 3,200 RPM.
    • The engine load is less than 60 percent.
    • An engine misfire is detected, and DTC P0300 is set.

Conditions for Setting the DTC

A serial data malfunction exists preventing the PCM from receiving rough road detection data for 20 seconds or more.

Action Taken When the DTC Sets

    • The control module stores the DTC information into memory when the diagnostic runs and fails.
    • The malfunction indicator lamp (MIL) will not illuminate.
    • The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Failure Records.
    • The driver information center, if equipped, may display a message.

Conditions for Clearing the DTC

    • A current DTC Last Test Failed clears when the diagnostic runs and passes.
    • A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other non-emission related diagnostic.
    • Clear the DTC with a scan tool.

Diagnostic Aids

    •  If you cannot duplicate the DTC, use the Freeze Frame/Failure Records in order to locate an intermittent condition in the following ways:
       - Use the information included in the Freeze Frame/Failure Records in order to determine how many miles since the DTC set.
       - Use the Fail Counter and Pass Counter in order to determine how many ignition cycles the diagnostic test reported as a pass or a fail.
       - Operate the vehicle within the same Freeze Frame conditions that you observed in order to isolate when the DTC failed. Use the following conditions for your tests:
   • The RPM
   • The load
   • The vehicle speed
   • The temperature
    • For an intermittent condition, refer to Intermittent Conditions .

Test Description

The number below refers to the step number on the diagnostic table.

  1. Service the ABS before diagnosing for a misfire, because an actual engine misfire may or may not exist. The following 2 examples illustrate how this DTC can be set with or without an actual engine misfire:

  2. • There was an actual engine misfire during an ABS malfunction.
    • The vehicle operator drove on a rough road, the ABS could not detect this due to a malfunction, and the PCM interpreted the variations in crankshaft speed caused by the rough road as a misfire.

Step

Action

Yes

No

1

Did you perform the Diagnostic System Check-Engine Controls?

Go to Diagnostic System Check - ABS

Go to Diagnostic System Check - Engine Controls