For 1990-2009 cars

### Circuit Description

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

### Conditions for Setting the DTC

• | Vehicle speed above 10 mph.

• | Engine speed below 3500 RPM.

• | Light to moderate engine load.

• | Engine misfire detected (DTC P0300 set).

• | A serial data malfunction exists preventing the PCM from receiving rough road detection data.

• | Conditions met for 20 seconds.

### Action Taken When the DTC Sets

• | The PCM will illuminate the Malfunction Indicator Lamp (MIL) after two consecutive drive trips that the misfire diagnostic runs and fails. This DTC can only set if the MIL is already ON for misfire.

• | The PCM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame and/or Failure Records.

### Conditions for Clearing the MIL/DTC

• | The PCM will turn the MIL OFF after three consecutive drive trips that the misfire diagnostic runs and does not fail.

• | If no failures are reported by this or any other non-emission related diagnostic.

• | A last test failed (Current DTC) will clear when the diagnostic runs and does not fail.

• | PCM battery voltage is interrupted.

• | Using a Scan tool.

### Diagnostic Aids

Using Freeze Frame and/or Failure Records data may aid in locating an intermittent condition. If the DTC cannot be duplicated, the information included in the Freeze Frame and/or Failure Records data can be useful in determining how many miles since the DTC set. The Fail Counter and Pass Counter can also be used to determine how many ignition cycles the diagnostic reported a pass and/or a fail. Operate vehicle within the same freeze frame conditions (RPM, load, vehicle speed, temperature etc.) that were noted. This will isolate when the DTC failed. For intermittents, refer to Symptoms .

### Test Description

Number(s) below refer to step numbers on the diagnostic table.

1. The serial data circuit (UART) must be serviced before diagnosing for misfire, because actual engine misfire may or may not exist. The following two examples illustrate how this DTC can be set with or without actual engine misfire:

• | There was actual engine misfire during a serial data circuit malfunction.

• | The vehicle was driven on a rough road, the PCM could not receive this data from the ABS system due to a serial data circuit malfunction, and the PCM interpreted the variations in crankshaft speed caused by the rough road as misfire.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | Go to Brakes, Antilock Brake System (ABS) | Go to Powertrain On Board Diagnostic (OBD) System Check