Chart A-2 No ALDL Data or Won't Flash Code 12

Diagnostic Circuit:

Circuit Description:
There should always be a steady "Service Engine Soon" light when the ignition is "ON" and engine stopped. Ignition is supplied directly to the light bulb. The Powertrain Control Module (PCM) will turn the light "ON" by grounding CKT 419 at the PCM.

With the diagnostic terminal grounded, the light should flash a Code 12, followed by any trouble code(s) stored in memory.

A steady light suggests a short to ground in the light control CKT 419.

Diagnostic Chart A-2:

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

1. If the light goes "OFF" when the PCM connector is disconnected, then CKT 419 is not shorted to ground.

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

3. At this point, the "Service Engine Soon" light wiring is OK. The problem is a faulty PCM or MEM-CAL. If Code 12 does not flash, the PCM should be replaced using the original MEM-CAL. Replace the MEM-CAL only after trying a PCM, as a defective MEM-CAL is an unlikely cause of the problem.