Malfunction Indicator Lamp (MIL) Always On

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 Alternative Fuels ECM connects into the normal MIL Control circuit path and does not affect the gasoline control module operation of the MIL. The Alternative Fuels ECM is connected into the MIL Control circuit path in order to command the MIL ON when a CNG DTC sets. Both the Alternative Fuels ECM 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 Alternative Fuels ECM can supply the ground path.

DIAGNOSTIC AIDS

Intermittent Condition
A fault not present indicates the condition that caused the symptom is intermittent and not currently present. Refer to the Intermittent Conditions diagnostic information located in Symptoms - Computers and Control Systems. If any DTCs were stored proceed to the applicable DTC table in the order stated in Diagnostic System Check - Alternative Fuels (Computers and Control Systems). Initial Inspection and Diagnostic Overview Intermittent Conditions

TEST DESCRIPTION

Steps 1-9:

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 tests if the MIL Input circuit is requesting MIL illumination. There should not be a MIL Input while the MIL is commanded OFF.