GM Service Manual Online
For 1990-2009 cars only

Diagnostic Instructions

    • Perform the Diagnostic System Check - Vehicle 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.

DTC Descriptors

DTC P1660: Malfunction Indicator Lamp (MIL) Control Circuit High Voltage

DTC P1661: Malfunction Indicator Lamp (MIL) Control Circuit Low Voltage

Diagnostic Fault Information

Circuit

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

MIL Voltage Supply

P1661

P1661

--

--

MIL Control

P1661/MIL ON

P1661

P1660

--

Circuit/System Description

The malfunction indicator lamp (MIL) is located on the instrument panel cluster (IPC). The MIL informs the driver that an emission system fault has occurred and that the engine control system requires service. The control module monitors the MIL control circuit for conditions that are incorrect for the commanded states of the MIL.

Conditions for Running the DTC

    • The ignition is ON or the engine is operating.
    • The DTCs runs continuously when the above conditions are met.

Conditions for Setting the DTC

P1660

The engine control module (ECM) detects the MIL control circuit is shorted to voltage.

P1661

The ECM detects the MIL control circuit is open or shorted to ground.

Action Taken When the DTC Sets

DTC P1660 and P1661 are Type B DTCs.

Conditions for Clearing the DTC

DTC P1660 and P1661 are Type B DTCs.

Reference Information

Schematic Reference

Engine Controls Schematics

Electrical Information Reference

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

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Verification

  1. Ignition ON, command the MIL ON and OFF with a scan tool. The MIL should turn ON and OFF when commanded.
  2. Operate the vehicle within the Conditions for Running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

Circuit/System Testing

  1. If any other IPC indicators or gages are inoperative, refer to Symptoms - Displays and Gages.
  2. Ignition OFF, disconnect the harness connector at the ECM.
  3. Ignition ON, connect a 3A fused jumper wire between the MIL control circuit terminal 39 and ground. The MIL should illuminate.
  4. If the MIL does not illuminate, test the control circuit for a short to voltage, short to ground, or an open/high resistance. If the circuit tests normal, replace the IPC/bulb.
    If the MIL illuminates, inspect for a poor connection or replace the ECM.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Instrument Cluster Bulb Replacement
    •  Instrument Cluster Replacement
    •  Control Module References for engine control module replacement, setup, and programming