What it means
The control circuit of the main ECM/PCM power relay is open - the ECU can lose power on shutdown sequence or fail to wake up at next start.
At a glance
- Usually lights the check-engine light
How it works
The ECM or PCM power relay switches battery power to the engine controller, and the controller monitors the relay’s control circuit. P0685 sets when that control path is open, which can prevent the relay from responding as commanded and leave the controller without power.
Common causes
Most likely first.
- Open in relay control or harness
- Pin pushed back out of connector body
- Cracked solder joint inside the module or sensor
Symptoms
- Check engine light on
- Possible limp mode or no-start
- Multiple follow-up faults across modules
- Reset/relearn may be needed after repair
How to diagnose it
- Record freeze-frame data and scan all modules for related low-voltage, communication, or relay codes before clearing anything.
- Check whether the engine controller wakes up reliably and note any intermittent no-start or shutdown symptoms.
- Inspect the power relay, its socket, and the control harness for damage, corrosion, loose fit, or a pushed-back terminal.
- Check relay control-circuit continuity and for unintended opens using the manufacturer’s wiring diagram and test procedure.
- Test the relay and verify controller power and ground during key-on and shutdown before suspecting an internal module fault.
Common fixes
- Seat or repair a pushed-back or loose relay-socket terminal.
- Repair an open or damaged relay-control wire.
- Clean corroded connections or replace a damaged relay socket.
- Replace a relay that fails the manufacturer’s test.
- Repair or replace the controller only after the circuit and relay have passed testing.
Can I keep driving?
A vehicle with this fault can stall, fail to start, or lose engine control power unexpectedly, so avoid unnecessary driving and stop if it cuts out. A repair may require clearing related faults or completing specified relearn procedures.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.