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

Chart A-2:

ECM Circuit:

CIRCUIT DESCRIPTION
There should always be a steady Malfunction Indicator Lamp (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 in the ECM.

With the Data Link Connector (DLC) diagnostic "test" terminal grounded, the light should flash a Diagnostic Trouble Code (DTC) 12, followed by any DTC(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 display serial data, 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 tool is functioning properly and CKT 800 is OK, the Programmable Read Only Memory (PROM) or ECM may be at fault for the no DLC symptom.

2. If the MIL 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.