Malfunction Indicator Lamp (MIL) Inoperative

Malfunction Indicator Lamp (MIL) Inoperative:

CIRCUIT DESCRIPTION
The Malfunction Indicator Lamp (MIL) is used to indicate to the driver of the vehicle that an engine control system condition has occurred that may cause an increase in emissions and/or cause a driveability concern.

When the ignition is first turned ON the gasoline control module (VCM/PCM or ECM) will illuminate the MIL for a bulb and system check. When the engine is started and operating the gasoline control module will turn the MIL OFF if no DTCs are set.

The AF ECU connects into the normal MIL Control circuit path and does not affect the gasoline control module operation of the MIL. The AF ECU is connected into the MIL Control circuit path in order to command the MIL ON when a CNG DTC sets. Both the AF ECU and the gasoline control module simultaneously test certain DTCs regardless of the fuel system that is in operation and may illuminate the MIL.

The instrument panel gauge fuse supplies the positive voltage source for the MIL. The gasoline control module and/or the AF ECU can supply the ground path.

TEST DESCRIPTION

Steps 1-3:

Steps 4-14:

Steps 15-17:

The numbers below refer to the step numbers on the diagnostic table.
1. The System Check MUST be performed before proceeding with this diagnostic table. Failure to perform the System Check will result in misdiagnosis.
2. With the ignition ON and the engine OFF the gasoline control module (VCM/PCM or ECM) should continuously illuminate the MIL.
3. The scan tool will command BOTH the gasoline control module and the AF ECU to turn the MIL OFF.
5. This step checks if a gasoline control module system failure is the cause of the inoperative MIL. The most likely cause of no MIL and a no start is a loss of voltage to the gasoline control module.
7. This step tests for an open MIL Input circuit. The gasoline control module should always command the MIL ON when the ignition is ON and the engine is OFF.
8. This step tests for an open MIL Control circuit or IP.