Chart A2 - No Code 12, "Check Engine" Light On Steady

The "CHECK ENGINE" light should come on when ignition is turned "ON" and engine is not running. Battery voltage is directly supplied to bulb. The ECM controls the light by grounding circuit 419. With diagnostic terminal grounded, light should flash Code 12, followed by other trouble codes stored in memory. A steady light indicates a grounded circuit 419 or an open diagnostic circuit 451.

NOTE: The following step numbers refer to the step numbers in the accompanying Flow Chart.

If the light goes off when the ECM connector is disconnected, then circuit 419 is not shorted to ground. Check connector terminals physically for proper contact.
This step checks for an open diagnostic circuit 451.
At this point wiring is okay, problem may be a faulty ECM or PROM. 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 1 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.

Fig 1: Chart A2 Schematic, No Code 12 ("CHECK ENGINE" Light On Steady)

Fig 2: Flow Chart A2, No Code 12 ("CHECK ENGINE" Light On Steady)

Fig 3: Flow Chart A2, No Code 12 ("CHECK ENGINE" Light On Steady)