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For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2005 🢒 TopKick C-Series (Conventional) C4/C5 🢒 Vehicle Control Systems 🢒 Vehicle DTC Information 🢒 Malfunction Indicator Lamp (MIL) Always On

Circuit Description

Ignition voltage is supplied to the malfunction indicator lamp (MIL). The engine control module (ECM) turns the MIL ON by grounding the MIL control circuit.

MIL Operation

The MIL is located on the instrument panel (I/P).

MIL Function

    • The MIL informs the driver that a malfunction has occurred and the vehicle should be taken in for service as soon as possible.
    • The MIL illuminates during a bulb test and a system test.
    • A DTC will be stored if a MIL is requested by the ECM.

MIL Illumination

    • The MIL will illuminate with ignition switch ON and the engine not running.
    • The MIL will turn OFF when the engine is started.
    • The MIL will remain ON if the self-diagnostic system has detected a malfunction.
    • The MIL may turn OFF if the malfunction is not present.
    • If the MIL is illuminated and then the engine stalls, the MIL will remain illuminated so long as the ignition switch is ON.
    • If the MIL is not illuminated and the engine stalls, the MIL will not illuminate until the ignition switch is cycled OFF, then ON.

Diagnostic Aids

If the problem is intermittent, refer to Intermittent Conditions .

Test Description

The number below refers to the step number on the diagnostic table.

  1. This step determines if the condition is with the MIL control circuit or the ECM.

Malfunction Indicator Lamp (MIL) Always On

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine Control Module Connector End Views or Engine Controls Connector End Views

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle in Vehicle DTC Information

2

  1. Turn OFF the ignition.
  2. Disconnect the engine control module (ECM).
  3. Turn ON the ignition, with the engine OFF.
  4. Observe the malfunction indicator lamp (MIL).

Is the MIL illuminated?

Go to Step 3

Go to Step 5

3
  1. Remove the instrument panel cluster (IPC). Refer to Instrument Panel Carrier Replacement in Instrument Panel, Gages, and Console.
  2. Test the MIL control circuit for a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 6

Go to Step 4

4

Replace the IPC. Refer to Instrument Panel Carrier Replacement in Instrument Panel, Gages, and Console.

Did you complete the replacement?

Go to Step 6

--

5

Replace the ECM. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming.

Did you complete the replacement?

Go to Step 6

--

6
  1. Turn the ignition OFF for 30 seconds.
  2. Start the engine.

Does the vehicle operate correctly without any MIL illumination, and without any stored DTCs?

System OK

Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information