Chart A-2 Won't Flash Code 12 Service Engine Light On

Chart A-2 - Won't Flash DTC 12. MIL (Service Engine Soon) "ON" Steady:

On-Board Diagnostic Wiring Circuit:

Circuit Description:

There should always be a steady MIL (Service Engine Soon) 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 DTC 12, followed by any diagnostic trouble code(s) stored in memory.

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

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 MIL (Service Engine Soon) wiring is OK. The problem is a faulty PCM or PROM. If DTC 12 does not flash, the PCM should be replaced using the original PROM. Replace the PROM only after trying an PCM, as a defective PROM is an unlikely cause of the problem.