Chart A-2 Code 12 Will Not Flash

Fig. 004 - Wiring Diagram for Chart A-2 Code 12 Will Not Flash. VIN 3, VIN 9:

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

There should always be a steady "SERVICE ENGINE SOON" Light when the ignition is "on" and engine stopped. Battery is supplied directly to the light bulb. The Electronic Control Module (ECM) will control the light and turn it on by providing a ground path through circuit 419 to 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.

Fig. 003 - Chart A-2 - Code 12 Will Not Flash:

1. If the light goes off when the ECM connector is disconnected, circuit 419 is not shorted to ground. Take this opportunity to physically check the connector terminals for proper contact.
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 are turned "ON" or "OFF" by the ECM internal electronic switches called "drivers". Each driver is part of a group of four called "Quad-Drivers". Failure of one can damage any other driver within the set.
Solenoid coil resistance must measure more than 20 ohms. Less resistance will cause early failure of the ECM "DRIVER". Using an ohmmeter, check the solenoid coil resistance of the following before installing a replacement ECM.
a. Canister Purge
b. A/C Relay
c. EGR Solenoid