Chart A-2 No DLC Data or Won't Flash DTC 12

Chart A-2 - No DLC Data Or Won't Flash DTC 12. Malfunction Indicator Lamp "ON" Steady:

"Service Engine Soon" Lamp Wiring 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 directly 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 Codes (DTCs) stored in memory.

A steady light 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, then the ECM should not flash a DTC 12. 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 light goes "OFF," when the ECM connector 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 (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.