Chart A-2 Won't Display Code 12 MIL Light On Steady

Chart A-2:

PCM Power Supply And Diagnostic Circuit Diagram:

CIRCUIT DESCRIPTION
There should always be a steady MIL (CHECK ENGINE) light when the ignition is "ON" and engine stopped. Battery voltage is supplied to the light bulb. The ECM will turn the light "ON" by grounding circuit 419 at the ECM. With the diagnostic terminal grounded, the light should flash a DTC 12, followed by any DTC's stored in memory. A steady light indicates a short to ground in the light control circuit 419, or an open in diagnostic circuit 451.

TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.

1. If there is a problem with the ECM that causes a "Scan" tool to not read Serial data, then the ECM should not flash DTC 12. If a DTC 12 does flash, be sure that the "Scan" tool is working properly on another vehicle. If the "Scan" tool is functioning properly and circuit 461 is OK, the EEPROM or ECM may be at fault for the "NO" ALDL symptom.

2. If the light goes "OFF" when the ECM connector is disconnected, then circuit 419 is not shorted to ground.

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

4. At this point the MIL wiring is OK. The problem is a faulty ECM. If a DTC 12 does not flash, the ECM should be replaced and reprogrammed using the original EEPROM.