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DTC codes / GM codes / P1671

P1671 · GMMIL Circuit Fault

P1671 on a GM vehicle means the powertrain control module (PCM) has detected a fault in the circuit that controls the malfunction indicator lamp (MIL), which is the check engine light on the dashboard. The PCM monitors the voltage on the MIL control circuit and sets this code when the voltage is outside the expected range, indicating an open, short, or excessive resistance in the wiring or the lamp driver inside the PCM.

SystemPowertrain
Code typeGM specific
SeverityMedium
Fix difficultyIntermediate
AreaComputer and output circuits

Symptoms you may notice

  • The check engine light may not illuminate when the ignition is turned on, or it may stay on constantly regardless of faults.
  • The bulb may appear fine but the PCM cannot command it on or off, so other diagnostic trouble codes may be present without the light showing.
  • In some cases, the light works normally and the only sign is the stored P1671 code during a scan.
  • Other dashboard warning lights or instrument cluster functions may be affected if the fault involves shared wiring.
  • The engine may run normally because this code does not directly affect drivability.
  • A failed state inspection if the MIL does not illuminate during the bulb check.

Common causes, most likely first

  1. Failed or damaged instrument cluster bulb or light-emitting diode (LED) for the check engine light.
  2. Open or shorted wiring between the PCM and the instrument cluster on the MIL control circuit.
  3. Corroded or loose electrical connectors at the PCM or cluster.
  4. Faulty PCM driver circuit for the MIL, though this is less common.
  5. Blown fuse or poor ground connection affecting the instrument cluster power or ground.
  6. Aftermarket alarm or remote start installation that tapped into the MIL circuit incorrectly.

How to diagnose and fix it

  1. Verify the check engine light bulb or LED by performing a bulb check: turn the ignition on and see if the MIL illuminates briefly; if not, inspect the bulb.
  2. Use a scan tool to command the MIL on and off to confirm whether the PCM can control the lamp.
  3. Check the wiring and connectors between the PCM and the instrument cluster for continuity, shorts to ground, and shorts to voltage.
  4. Inspect the instrument cluster for cold solder joints or failed driver circuits, especially on high-mileage vehicles.
  5. Test the PCM MIL driver output with a test light or oscilloscope to confirm the PCM can ground the circuit.
  6. Repair any damaged wiring or replace the instrument cluster or PCM as needed, then clear the code and verify the MIL operates correctly.

When the check engine light lies

  • A P1671 code often means the check engine light itself is not working, so other emissions-related faults may go unnoticed and the vehicle could fail an inspection without warning.
  • Technicians sometimes replace the PCM unnecessarily when the real problem is a burned-out bulb or a bad connection at the cluster, so always test the lamp circuit first.
  • On some GM vehicles, the MIL is controlled by the body control module (BCM) over a serial data bus, so a communication fault or BCM issue can set P1671 even though the PCM is fine.
  • Aftermarket accessories like remote starters or alarm systems that splice into the instrument cluster wiring are a common cause of MIL circuit faults.
  • The code can be set by a loose ground at the instrument cluster or a poor connection at the PCM, so wiggle-testing connectors while monitoring the circuit is a smart diagnostic step.

Can I keep driving with P1671?

You can usually drive the vehicle with P1671 because it does not affect engine performance, but the check engine light may not work, so you could miss other serious faults. Avoid relying on the light for emissions testing and get the circuit repaired soon to ensure the MIL functions properly.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for GM vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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