Chart A-2 Won't Flash DTC 12 No Serial Data MIL On Steady

Chart A-2:

MIL Circuit:

CIRCUIT DESCRIPTION:
There should always be a steady MIL (Service Engine Soon) when the ignition is "ON" and engine stopped. ignition voltage is supplied to the light bulb. The Engine Control Module (ECM) will turn the light "ON" by grounding CKT 419 at the ECM.

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

A steady tight suggests a short to ground in the light control CKT 419 or an open in diagnostic CKT 451.

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 to not read serial data, the ECM should not flash a DTC 12. If DTC 12 is flashing check for CKT 451 short to ground. If DTC 12 does flash be sure that the Tech 1 scan tool is working properly on another vehicle. if the Tech 1 scan tool is functioning properly and CKT 461 is OK, the PROM or ECM may be at fault for the NO DLC symptom.

2. If the tight goes "OFF" when the ECM connector is disconnected, CKT 419 is not shorted to ground.

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

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