GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Generic 🢒 2008 🢒 Unit Repair 🢒 Engine 🢒 Engine Controls and Fuel - LS Engines 🢒 Malfunction Indicator Lamp (MIL) Diagnosis

Diagnostic Instructions
    • Perform the Powertrain On Board Diagnostic (OBD) System Check prior to using this diagnostic procedure.
    • Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provides an overview of each diagnostic category.

Circuit/System Description

Ignition 1 voltage is supplied directly to the malfunction indicator lamp (MIL) light emitting diode (LED). The engine control module (ECM) turns the MIL ON by grounding the MIL control circuit. There should be a steady MIL with the ignition ON and the engine OFF.

MIL Location

The MIL is located in the under hood fuse-block assembly.

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 the 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 no longer present.
    • If the MIL is illuminated and then the engine stalls, the MIL will remain illuminated as 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.

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

    •  Engine Control Module Connector End Views
    •  Engine Controls Connector End Views

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

Scan Tool Reference

Engine Control Module Scan Tool Information

Circuit/System Verification

Ignition ON, command the MIL ON and OFF with a scan tool. The MIL should turn ON and OFF when commanded.

Circuit/System Testing
  1. Ignition OFF, disconnect the MIL LED from the under hood fuse-block.
  2. Ignition ON, test for B+ between the ignition voltage circuit terminal and ground.
    3. ⇒If less than the specified range, repair the ignition voltage circuit for a short to ground, or an open/high resistance.
  3. Connect a test lamp between the MIL control circuit terminal and B+.
  4. Command the MIL ON and OFF with a scan tool. The test lamp should respond to the scan tool commands.
    5. ⇒If the MIL does not illuminate, test the MIL control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the ECM.
    ⇒If the MIL remains illuminated all the time, test the MIL control circuit for a short to ground. If the circuit tests normal, replace the ECM.
  5. If all circuits test normal, replace the MIL LED.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Engine Control Module Replacement for engine control module replacement, setup, and programming