Chart A2: Won't Flash Code 12, "SES" Light On
"SERVICE ENGINE SOON" (SES) light should be on steady when ignition is on and engine is off. 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 the light goes off when the ECM connector is disconnected, then circuit No. 419 is not shorted to ground.
- If there is a problem with the ECM that causes a "Scan" tester to not read serial data, the ECM should not flash a Code 12. If Code 12 does flash, be sure that the "Scan" tester is working properly on another vehicle. If the "Scan" tester is functioning properly and circuit No. 461 is okay, the MEMCAL or ECM may be at fault for the "NO ALDL" symptom.
- Checks for an open diagnostic circuit No. 451.
- At this point, the "SERVICE ENGINE SOON" light wiring is okay. The problem is a faulty ECM or MEMCAL. If Code 12 does not flash, the ECM should be replaced using the original MEMCAL. Replace the MEMCAL only after trying a new ECM, as a defective MEMCAL is an unlikely cause of the problem.