Chart A-2 Won't Flash Code 12 Service Engine Light On

Chart A-2 - Won't Flash Code 12 ("Service Engine Soon" Light On Steady):

Wiring Diagram For Chart A-2 - Won't Flash Code 12 ("Service Engine Soon" Light On Steady):

CHART A-2 - WON'T FLASH CODE 12 ("SERVICE ENGINE" LIGHT ON)

Circuit Description:

There should always be a steady "SERVICE ENGINE SOON" Light when the ignition is "on" and engine stopped. Battery ignition voltage is supplied to the light bulb. The Electronic Control Module (ECM) will turn the light on by grounding circuit 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 circuit 419, or an open in diagnostic circuit 451.


Test Description:

1. If the light goes off when the ECM connector is disconnected, then circuit 419 is not shorted to ground. Also, check the connector terminals physically for proper contact at this time.

2. This step will check for an open diagnostic circuit 451.

3. At this point, the "SERVICE ENGINE SOON" light wiring is okay. The problem is a faulty ECM or PROM. The ECM is OK if a Code 51 is stored when the PROM is removed. Replace the PROM.


4. Solenoids and relays are turned "ON" or "OFF" by the ECM using internal electronic switches called "drivers". Each driver is part of a group of four, called "Quad-Drivers". Failure of one driver can damage any other driver in the set. Before replacing ECM, be sure to check the coil resistance of all solenoids and relays controlled by the ECM. See ECM wiring diagram for the solenoid(s) and relay(s) and the coil terminal identification.