On-Board Diagnostics: Notes

The Engine Control Module (ECM - M/T) or Powertrain Control Module (PCM - A/T) monitors the input/output signals (some signals all the time and others under specified conditions) of the ECM (M/T) or PCM (A/T). When a malfunction continues for a specified time after the irregular signal is initially monitored, the ECM (M/T) or PCM (A/T) judges that a malfunction has occurred. After the ECM (M/T) or PCM (A/T) first detects a malfunction, a diagnostic trouble code is recorded when the engine is restarted and the same malfunction is re-detected. However, for some failures a diagnostic trouble code will be recorded at first detection, refer to DIAGNOSTIC TROUBLE CODE DEFINITIONS . The diagnostic results can be read out with a scan tool. Since memorization of the diagnostic trouble codes is backed up directly by the battery, the diagnostic results are memorized even if the ignition key is turned off. The diagnostic trouble codes will, however, be erased when the battery terminal or the ECM (M/T) or PCM (A/T) connector is disconnected. In addition, the diagnostic trouble code can also be erased by turning the ignition on and sending the diagnostic trouble code erase signal from scan tool MUT-II (MB991502) to the ECM (M/T) or PCM (A/T).

NOTE: If the sensor connector is disconnected with the ignition turned on, the diagnostic trouble code is memorized. In this case, send the diagnostic trouble code erase signal to the ECM (M/T) or PCM (A/T) in order to erase the diagnostic memory.

The ECM (M/T) or PCM (A/T) records the engine operating condition when the diagnostic trouble code is set. This data is called "freeze frame" data. This data can be read by using the scan tool, and can then be used in simulation tests for troubleshooting.