Circuit Description
The Malfunction Indicator Lamp (MIL) should remain steady when the ignition is ON and engine is OFF. The battery ignition voltage is supplied to the MIL. The ECM turns the MIL ON by grounding the MIL control circuit.
With the diagnostic terminal grounded, the MIL should flash a DTC 12, followed by any DTCs stored in memory.
A steady light suggests a short to a ground in the MIL control circuit or an open in the diagnostic test circuit.
Diagnostic Aids
If you do not find any trouble in the fuel pump circuit or the ignition system, and you have not found the cause of the Engine Cranks But Will Not Run, check for the following problems:
| • | Fouled spark plugs |
| • | EGR valve stuck open |
| • | Low fuel pressure -- Refer to Fuel System Diagnosis . |
| • | Water or foreign material in the fuel system |
| • | A grounded IC control circuit may cause a No Start or a Start then Stall condition |
| • | Basic engine problem |
| • | IAC stuck open |
| • | Check the fuel filter (partially plugged) |
Test Description
The numbers below refer to the step numbers on the Diagnostic Table:
-
If there is a problem with the ECM that causes a scan tool to not read the engine data then the ECM should not flash a DTC 12. If a DTC 12 does flash, make sure that the scan tool works properly on another vehicle. If the scan functions properly and the engine data circuit is OK, the PROM or ECM may be at fault for the No Scan Data symptom.
-
If the lamp goes OFF when you disconnect the ECM connector, then the MIL control circuit is not shorted to a ground.
-
This step will check for an open diagnostic test circuit.
-
At this point the MIL wiring is OK. The problem is a faulty ECM or PROM. If DTC 12 does not flash, replace the ECM using the original PROM. Replace the PROM only after trying an ECM, as a defective PROM is an unlikely cause of the problem.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Did you perform the On-Board Diagnostic (OBD) System Check? | -- | ||
2 | Turn the ignition ON leaving the engine OFF. Does the MIL lamp come ON? | -- | ||
Ground the diagnostic test circuit at the Data Link Connector (DLC). Does the MIL flash DTC 12? | -- | |||
4 |
Did you find a problem? | -- | ||
5 |
Did you find a problem? | -- | ||
6 |
Did you find a problem? | -- | ||
Does the MIL turn ON? | -- | |||
8 |
Is the action complete? | -- | -- | |
Does the MIL flash DTC 12? | -- | |||
Did you find a problem? | -- | |||
11 | Replace the ECM using the original PROM. Refer to Engine Control Module Replacement . Does the MIL flash DTC 12? | -- | ||
12 | Replace the PROM. Refer to PROM Replacement Is the action complete? | -- | -- | |
13 |
Did you find a problem? | -- | ||
14 | Repair the open in the system ground between the DLC and the ECM ground. Is the action complete? | -- | -- | |
15 | Replace the ECM. Refer to Engine Control Module Replacement . Is the action complete? | -- | -- | |
16 |
Does the engine start and continue to run? | -- | ||
17 | Allow the engine to idle until it reaches the normal operating temperature. Are any DTCs stored? | -- | Go to the Applicable DTC Table | System OK |
