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P0685ECM/PCM Power Relay Control Circuit/Open

The engine computer has detected an open circuit in the control side of its main power relay, which supplies switched battery power to the PCM and many engine actuators.

Definition

The PCM energizes its main power relay (often called the ECM/PCM power relay or main relay) by driving the relay coil circuit, and it monitors that circuit for faults. P0685 sets when the control circuit reads open — the coil, its feed fuse, the wiring, or the PCM driver is not completing the circuit. Because this relay feeds the PCM and commonly the injectors, coils, and sensors, symptoms range from an intermittent stall or crank-no-start to a dead engine with multiple modules offline, along with a MIL when the engine does run. An engine that can stall without warning makes this a priority repair.

Common causes

  • Failed main power relay (open coil or burned socket terminals)
  • Blown fuse feeding the relay coil circuit
  • Corroded or heat-damaged terminals in the underhood fuse block (a known GM fuse-block failure pattern)
  • Open wiring between the PCM and the relay coil
  • Worn ignition switch not feeding the circuit
  • Failed PCM relay driver (rare)

Common misdiagnoses

  • PCM replacement when the relay or fuse block is at fault
  • Battery replacement for the no-start symptom
  • Starter replacement for what is a power-supply stall/no-start
  • Ignition switch replacement without testing the relay circuit first

Troubleshooting steps

  1. 1.Check the relay and coil-feed fuses

    Verify every fuse in the ECM/PCM relay circuit with a test light or meter rather than by eye.

  2. 2.Swap or bench-test the relay

    Substitute an identical known-good relay and measure the original coil — typically 60-120 ohms; an open coil confirms the relay.

  3. 3.Test the relay socket

    With the key on, confirm battery voltage on the coil feed terminal and check the socket for burned, loose, or corroded contacts.

  4. 4.Check the PCM control wire

    Verify continuity of the coil control circuit between the relay socket and the PCM connector.

  5. 5.Inspect the fuse block itself

    Look for heat discoloration and spread terminal cavities; some vehicles require fuse-block terminal or block replacement.

  6. 6.Confirm the repair under vibration and heat

    Idle and drive the vehicle to operating temperature while monitoring for relay dropout, since these faults are often intermittent.

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