System Description

The Powertrain Control Module (PCM) monitors many different circuits in the fuel injection system. If the PCM senses a problem with a monitored circuit often enough to indicate an actual problem, it stores a diagnostic trouble code in the PCM's memory.

After the PCM first detects a malfunction, a diagnostic trouble code is recorded when the engine is restarted and the same malfunction is re-detected. However, for items marked with a "*" a diagnostic trouble code is recorded on the first detection of the malfunction.

After that, if the PCM does not re-detect the malfunction for 40 drives* (51 engine start for non-emission related faults), the diagnostic trouble code will be erased from the PCM memory.

*: A drive indicates from engine start to stop and monitors the power train component.

However, for misfiring or a fuel system rich/lean, the diagnostic trouble codes will be erased under the following conditions.
- When driving conditions (engine speed, engine coolant temperature, etc.) are similar to those when the malfunction was first recorded.
- When the PCM does not re-detect the malfunction for 80 drives*.

Technicians can display stored diagnostic trouble codes by two different methods.
- The first is to cycle the ignition switch On-Off-On-Off-On within 5 seconds. Then count the number of times the malfunction indicator lamp (check engine lamp) on the instrument panel flashes on and off. The number of flashes represents the diagnostic trouble code. There is a slight pause between the flashes representing the first and second digits of the code. Longer pauses separate individual trouble codes.
- The second method of reading diagnostic trouble codes uses the scan tool. Connect the scan tool to the data link (diagnostic) connector in the vehicle.

FREEZE FRAME DATA
The PCM records the diagnostic trouble code and also the engine operating conditions at the time the malfunction was detected. These data are called "freeze frame" data.

These data indicate the engine operating condition from when no problems are detected to the initial detection of the malfunction. However, misfiring or fuel trim malfunction data are always replaced with the latest data.

These data can be read by using the scan tool, and can then be used in simulation tests for troubleshooting.

Data Items Recorded Are:
- Engine coolant temperature (C or F)
- Engine speed (r/min or RPM)
- Vehicle speed (km/h or mph)
- Long-term fuel compensation (Long-term fuel trim %)
- Short-term fuel compensation (Short-term fuel trim %)
- Fuel control condition
^ Open loop
^ Closed loop
^ Open loop-Drive condition
^ Open loop-DTC set
^ Malfunction of closed loop-O2 (rear)
- Calculated load value (%)
- MAP vacuum (in.Hg vacuum)
- Diagnostic trouble code during data recording