On-Board Diagnostics

ON-BOARD DIAGNOSTICS
The engine control module (ECM) or powertrain control module (PCM) monitors the input/output signals (some signals all the time and others under specified conditions) of the ECM or PCM . When a malfunction continues for a specified time or longer after the irregular signal is initially monitored, the ECM or PCM judges that a malfunction has occurred. After the ECM or 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. There are 51 diagnostic items. 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 or PCM connector is disconnected. In addition, the diagnostic trouble code can also be erased by turning the ignition switch to ON and sending the diagnostic trouble code erase signal from scan tool MUT-II (MB991502) to the ECM or PCM .

NOTE: If the sensor connector is disconnected with the ignition switch turned on, the diagnostic trouble code is memorized. In this case, send the diagnostic trouble code erase signal to the ECM or PCM in order to erase the diagnostic memory The 51 diagnostic items are all indicated sequentially from the smallest code number. The ECM or PCM records the engine operating condition when the diagnostic trouble code is set. This data is called "Freeze-frame" date. This data can be read by using the scan tool and can then be used in simulation tests for troubleshooting. Data items are as follows: