Chart A-2 No Codes Service Engine Soon Light ON Steady

Chart A-2 - Code 12 Will Not Flash:

Wiring Diagram for No "Service Engine Soon" Light.:

CHART A-2 NO ALDL DATA OR WON'T FLASH CODE 12 "SERVICE ENGINE SOON ON" STEADY


Circuit Description:
There should always be a steady "Service Engine Soon" (SES) light when the ignition is "ON" and engine stopped. Battery 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 Electronic Control Module (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 451 and ECM terminal A-8 are OK, the PROM or ECM may be at fault for the NO Assembly Line Data Link (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 "Service Engine Soon" light wiring is OK. The problem is a faulty ECM or Programmable Read Only Memory (PROM). If Code 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.