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

Chart A-2:

Malfunction Indicator Lamp Wiring Circuit:

Circuit Description:
There should always be a steady MIL (Check Engine) when the ignition is "ON" and the engine is "OFF." Battery voltage is supplied directly to the lamp. The Engine Control Module (ECM) will control the lamp and turn it "ON" by providing a ground path through CKT 419 to the ECM.

With the diagnostic terminal grounded, the lamp should flash a Diagnostic Trouble Code (DTC) 12, followed by any DTC(s) stored in memory. A steady lamp suggests a short to ground in the lamp control CKT 419, or an open in diagnostic CKT 451.

Chart 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 data from the ECM, then the 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 461 is OK. the PROM or ECM may be at fault for the "No Data Link Connector Data" symptom.
2. If the lamp turns "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 (Check Engine) wiring is OK. The problem is a faulty ECM. If DTC 12 does not flash, the ECM should be replaced.