Won't Flash Code 12 "Service Engine Soon" ("SES") Light On
The "SERVICE ENGINE SOON" light should come on when ignition is turned on and engine is not running. Battery voltage is supplied directly to light bulb. The ECM controls the light by grounding circuit No. 419. With diagnostic terminal grounded, light should flash Code 12, followed by other trouble codes stored in memory. A steady light indicates a short to ground in circuit No. 419, or an open diagnostic circuit No. 451.
NOTE:
The following step numbers refer to the numbers in the accompanying flow chart(s).
- If light goes off when ECM connector is disconnected, then circuit No. 419 is not shorted to ground. Check connector terminals physically for proper contact.
- This step checks for an open diagnostic circuit No. 451.
- At this point wiring is okay, problem may be a faulty ECM or PROM. The ECM is okay if Code 51 is stored when PROM is removed. Replace PROM.
- Solenoids and relays are turned on or off by ECM, using electronic switches called "drivers". Each driver is part of a group of 4, called "Quad Drivers". Failure of one driver can damage other drivers in set. Solenoid and relay coil resistance must be more than 20 ohms, as less resistance will cause early failure of ECM drivers. Before replacing ECM, be sure to check coil resistance of all solenoids and relays controlled by ECM. When checking Torque Converter Clutch (TCC) solenoid, be sure to raise drive wheels and run vehicle above 30 MPH to close 3rd gear switch, if used.