- Description of On-Board Diagnostics

IMPORTANT: When the diagnostic system is switched from Normal Mode to Check Mode or, from Check Mode to Normal Mode, it will erase all the DTC(s), Freeze Frame Records and System Status Flags (I/M readiness) information that was stored in the ECM/PCM. This will also occur if the vehicle is in the Check Mode and the ignition switch is turn to the "OFF" position. Before switching into either diagnostic mode always check the DTC, and Freeze Frame data, and note them down.

CHECK MODE DESCRIPTION
When the vehicle is in the Check Mode the ECM/PCM has an increased sensitivity to detect a malfunction. This function is particularly useful in diagnosing intermittent problems or conditions which are difficult to duplicate.

The same diagnostic items which are detected in Check Mode can also be detected in the Normal Mode.

CHECK MODE FUNCTION
The Check Mode function can be found under Misc. Tests of the scan tool. Check Mode can only be enabled with the ignition switch in the "ON" position and the engine NOT running. Once the vehicle is in the Check Mode the MIL will start to flash rapidly. This will help indicate that the vehicle is in the Check Mode, it can also be verified by using the scan tool, under Engine Data there is a parameter called Diagnostic Mode. It will display either Normal Mode or Check Mode. The MIL will illuminate if a fault is detected and a DTC is set. Once the engine has been started the MIL will stop flashing when in Check Mode. The vehicle will stay in Check Mode until either the scan tool requests the ECM/PCM back to Normal Mode or the ignition switch is turn to the "OFF" position.

NOTICE: When performing Check Mode diagnostics, follow the service manual procedure carefully. This will prevent setting false codes and misinterpreting any codes that have been set.