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

CIRCUIT DESCRIPTION
There should always be a steady "Check Engine" light when the ignition is "ON" and the engine stopped. Battery voltage is supplied directly to the light bulb. The electronic 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 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 or an open in diagnostic CKT 451.

TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.
1. If there is a problem with the ECM that causes a "Scan" tool to not read serial data, then the ECM should not flash a Code 12. If Code 12 does flash, be sure that the "Scan" tool is working properly on another vehicle. If the "Scan" is functioning properly and CKT 461 is OK, the Mem-Cal or ECM may be at fault for the no ALDL 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 "Check Engine" light wiring is OK. The problem is a faulty ECM or Mem-Cal. If Code 12 does not flash, the ECM should be replaced using the original Mem-Cal. Replace the Mem-Cal only after trying an ECM, as a defective Mem-Cal is an unlikely cause of the problem.