Engine Control Module - ECM

A-2 Chart:

MIL Circuit:

Circuit Description:
There should always be a steady MIL when the ignition is "ON" and engine stopped. Battery voltage is supplied to the lamp. The ECM will turn the lamp "ON" by grounding CKT 419 at the ECM.
With the diagnostic terminal grounded, the lamp should flash a DTC 12, followed by any DTC(s) stored in memory.
A steady lamp suggests a short to ground in the lamp control CKT 419, or an open in diagnostic CKT 451.

Chart Test Description: Number(s) below refer to circled number(s) on the diagnostic chart.

1. If there is a problem with the ECM that causes a Tech 1 scan tool not to read serial data, then the ECM should not flash a DTC 12. If DTC 12 does flash, be sure the scan tool is working properly on another vehicle. If the scan is functioning properly and CKT 461 is OK, the PROM or ECM may be at fault for the No DLC symptom.

2. If the lamp goes "OFF" when the DLC is disconnected, then CKT 419 is not shorted to ground.

3. This step will check for an open diagnostic CKT 451.

4. At this point the MIL wiring is OK. The problem is a faulty ECM or PROM (CAL-PAK). If DTC 12 does not flash, the ECM should be replaced using the original PROM (CAL-PAK). Replace the PROM (CAL-PAK) only after trying an ECM, as a defective PROM (CAL-PAK) is an unlikely cause of the problem.