P0685
PowertrainECM/PCM Power Relay Control Circuit Open
The wire that tells the main power relay to switch on and feed the engine computer has gone open circuit, so the ECU can't reliably control its own power supply. For you that usually shows up as a car that won't start, or one that starts then dies a few seconds later. It's an electrical fault rather than a mechanical one, and on most cars it traces back to a tired relay or a corroded connector rather than a dead computer.
ⓘ Information only. This page provides general educational information about fault code P0685. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0685 mean?
P0685 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: ECM/PCM Power Relay Control Circuit Open.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine cranks over fine but won't fire, or fires and then cuts out almost straight away
- • Stalling out of nowhere, often while driving or pulling away
- • Engine warning light on, sometimes with a cluster of other lit-up warnings at the same time
- • Throttle goes dead or the car drops into limp mode for no obvious reason
- • Dash lights flickering or electrical bits acting up when the relay supply is unstable
Possible causes
Causes commonly associated with P0685, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Worn or sticking ECM power relay, the relay contacts or coil pack in are simply on their way out. This is where the money is most of the time
- 2. Blown fuse on the ECM power feed, cheap to check and rule out before anything else
- 3. Corroded or loose terminals at the relay socket or the ECU connector, very common on cars that live outside or have had water in the engine bay
- 4. Chafed or broken wiring in the relay control circuit, often where the loom rubs against a bracket or edge over the years
- 5. Poor earth or corroded battery cable ends starving the circuit of a clean voltage reference
- 6. Faulty ignition switch not passing the switched feed through properly
- 7. A failed ECU itself, the last thing you should suspect, not the first
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Pull the codes and look at what else is stored alongside it, then check the live data while you crank. A P0606 or P0600 sitting with it points more at the ECU side than the relay
- 2. Get the relay out and look at it, then swap in a known-good one of the same rating. A spare from a non-critical circuit is often the same part number and proves it in two minutes
- 3. Check the relevant fuse and the relay socket for burnt or spread terminals, a relay that's been arcing leaves tell-tale browning
- 4. Back-probe the ECU power feed pin with the relay energised and look for a solid 12V or so. A reading that sags or drops in and out is your fault
- 5. Wiggle-test the loom and connectors while watching the voltage, intermittent opens nearly always show themselves when you move the wires by hand
- 6. If the relay, fuse and wiring all check out and you've still got an open on the control side, the ECU's internal relay driver is the remaining suspect
Common questions about P0685
What's the worst that happens if I just keep driving it? +
You risk being left at the side of the road. With this fault the ECU can lose its power feed at any moment, which means the engine can cut out mid-motorway or refuse to restart once you've stopped at the shops. It's not the kind of code that quietly worsens over months. An intermittent open on a power feed tends to go from occasional to constant fairly quickly, so it's not one to nurse along.
How quickly do I need to sort this? +
Treat it as a soon-as-you-can job, not a next-service job. If the car is starting and running normally for now you've got a little time to diagnose it properly, but if it's already cutting out or being awkward to start, stop driving it for anything important. The fault gives no warning before it strands you, so get it on a ramp or onto a known-good relay before you rely on the car again.
Is it the relay itself or the wiring and connectors? +
On the majority of cars it's the relay or its socket, and that's the cheapest outcome. Start there because a relay swap costs around £15 to £40 for the part and takes minutes. If a fresh relay doesn't clear it, the trouble is usually corroded terminals or a chafed wire in the control circuit. A dead ECU does cause this code but it's rare, so don't let anyone sell you a computer until the relay and wiring have been properly ruled out.
How long does the fix actually take? +
A relay or fuse swap is a ten-minute job once you've found the right one. Tracing and repairing a wiring fault is the variable one, anything from an hour to half a day at an independent garage depending on how buried the loom is. If it does turn out to be the ECU, you're looking at the part plus programming, which usually means a day with a dealer or auto-electrician and a bill into four figures.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →