GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2007 🢒 Sierra Classic - 2WD 🢒 Diagnostic Navigation 🢒 Vehicle Diagnostic Information 🢒 Malfunction Indicator Lamp (MIL) Always On

Circuit Description

Voltage is supplied directly to the malfunction indicator lamp (MIL). The powertrain control module (PCM) turns the MIL ON by grounding the MIL control circuit.

MIL Operation

The MIL is located on the instrument panel cluster (IPC).

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 diagnostic.

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 Testing for Intermittent Conditions and Poor Connections .

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 PCM.

Step

Action

Yes

No

Schematic Reference: Instrument Cluster Schematics or Engine Controls Schematics

Connector End View Reference: Displays and Gages Connector End Views or Powertrain Control Module Connector End Views

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

  1. Turn OFF the ignition.
  2. Disconnect the powertrain control module (PCM).
  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 Cluster Replacement .
  2. Test the MIL control circuit for a short to ground. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 6

Go to Step 4

4

Replace the IPC. Refer to Instrument Cluster Replacement .

Did you complete the replacement?

Go to Step 6

--

5

Replace the PCM. Refer to Control Module References 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.
  3. Observe the MIL.

Does the MIL operate correctly?

Go to Step 7

Go to Step 2

7

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

Go to Diagnostic Trouble Code (DTC) List - Vehicle

System OK