Electrical and sensors

P0685 — ECM/PCM Power Relay Control Circuit Open

Severity: medium Drive with care, to a shop

What P0685 means

P0685 sets when the engine control module sees an open or faulty signal in the control circuit of the main power relay. That relay supplies power to the ECM and often to injectors, ignition coils and other outputs once the ignition is on. If the circuit is open or the relay fails, power can drop out completely or intermittently.

Symptoms

  • Engine will not start
  • Engine stalls while driving or idling
  • Intermittent crank but no start
  • Check Engine light on
  • Fault comes and goes with vibration or temperature

Possible causes

  1. Failed main ECM power relay
  2. Open or poor connection in the relay control wire
  3. Blown fuse in the relay or ECM circuit
  4. Corroded or loose relay socket or connector terminals
  5. Damaged wiring harness, chafing or corrosion
  6. Internal ECM driver fault
  7. Poor ground at the ECM or relay

How to diagnose

  1. Scan for codes, review freeze frame data and look for other power-related codes
  2. Find the main ECM relay and its fuses in the wiring diagram and check them
  3. Inspect the relay socket for corrosion, heat damage and loose terminals
  4. Test the relay: listen for a click at key-on and measure the coil resistance
  5. Swap in a known-good relay of the same type
  6. Test the control wire from relay to ECM for opens and shorts
  7. Check for power and ground at the ECM connector with the ignition on
  8. If wiring is fine, have the ECM tested by a specialist

Common fixes

  • Replace the main ECM relay
  • Replace the fuse after finding why it blew
  • Repair or replace damaged wiring
  • Clean and re-crimp connector or relay socket terminals
  • Repair or replace the ECM if it has an internal fault

Questions and answers

What does P0685 mean?

It means the circuit controlling the ECM's main power relay is open. The module may get unstable power, so the engine sometimes starts and sometimes doesn't.

Can I drive with a P0685 code?

It's best not to. The engine can stall at any time. If you must drive, keep it short and only to get it repaired.

Will replacing the relay fix it?

Often, but not always. If the code returns, check the fuse, connector, control wire and grounds, then the ECM itself.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.