Chart A-2, No Code 12 "Service Engine Soon" Light On
"SERVICE ENGINE SOON" light should be on steady when ignition is on and engine is off. Battery voltage is supplied to the bulb. Bulb is grounded by ECM through circuit No. 419. With the diagnostic terminal grounded, the light should flash a Code 12, followed by any trouble codes stored in memory. A steady light is possibly a short to ground in circuit No. 419, or an open in diagnostic circuit No. 451.
NOTE:
Test numbers refer to test numbers on diagnostic chart.
- If there is a problem with the ECM that causes a "Scan" tester to not read serial data, then the ECM should not flash Code 12. If Code 12 does flash, be sure the "Scan" tester is functioning properly on another vehicle. If "Scan" tester is working properly, and circuit No. 461 is good, the MEM-CAL or ECM may be at fault.
- This step checks for shorted circuit No. 419 to ground.
- This step checks for an open circuit No. 451.
- At this point, light wiring is okay. Problem is a faulty ECM or MEM-CAL. ECM is okay if Code 51 is stored when PROM is removed.
If Code 12 does not flash, the ECM should be replaced using the original MEM-CAL. Replace the MEM-CAL only after trying a different ECM, as a defective MEM-CAL is an unlikely cause of this problem.