To enter the diagnostics, either connect a scan tool to the ALDL or use a jumper wire to connect terminals 1 and 3. Turn the ignition switch ON , then the ECM will enter the diagnostic program and report trouble Codes on the scan tool or by flashing the CHECK ENGINE light.
All Codes are 2 digits between 12 and 51. Codes are displayed on the CHECK ENGINE light by flashing the MIL with short and long pauses to distinguish digits of 1 code from digits of another. The short pause is used between digits of the same code, long pauses are between codes. For Code 12, the sequence will be: flash, pause, flash-flash, long pause.
- When the diagnostic mode is entered, Code 12 is displayed 3 or 4 times, depending on the vehicle. This indicates that the internal diagnostic system is operating. If Code 12 is not displayed, the self-diagnostic program is not functioning properly. If only Code 12 is displayed, no system malfunctions have been stored.
- After code 12 flashes to indicate diagnostic display, any existing system fault Codes are displayed in order from lowest to highest code number. Each Code is displayed 3 times, followed by the next Code, if any.
- When all engine and transaxle Codes have been displayed, Code 12 will flash again.