Won't Display DTC 12, MIL On Steady, Won't Display Data

DIAGNOSTIC CHART





WIRING DIAGRAM

INSPECTION PROCEDURE

Circuit Description:
There should always be a steady "Check Engine" MIL, 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 A5 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 suggests a short to ground in the light control CKT A5, or an open in diagnostic CKT A9.

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 Scan" tool to not read serial data, then ECM should not flash a DTC 12. If DTC 12 does flash, be sure that the "Scan" tool is working properly on another vehicle. If the "Scan" is functioning properly and CKT A8 is OK, the ECM may be at fault for the NO DLC symptom.
2. If the light goes "OFF", when the ECM connector is disconnected, then CKT A5 is not shorted to ground.
3. This step will check for an open diagnostic CKT A9.
4. At this point, the "Check Engine" ML wiring is OK. The problem is a faulty ECM or Programmable Read Only Memory (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.