P0685: ECM/PCM Power Relay Control Circuit/Open
A generic powertrain (engine and transmission) trouble code, same meaning on every brand.
If it starts and stays running, yes to a shop - but no-start episodes are the signature.
Symptoms
- Intermittent no-start with no crank issues
- Engine dies as if the key turned off
- Sometimes starts after wiggling the key or waiting
Likely causes, most common first
- Main relay worn contacts
- Relay socket or wiring corrosion
- Ignition switch feed faults
Read P0685 yourself before paying for diagnosis
A basic Bluetooth OBD-II scanner ($20-40) plugs into the port under the dashboard and shows this code with live sensor data on your phone - useful both for confirming the fix and for talking to a shop from an informed position.
What is going on and how it gets diagnosed
P0685 concerns the main power relay circuit that feeds the engine computer. When that feed drops, the engine dies instantly and cleanly - no sputter, just off - and typically restarts once the relay contacts cool or reseat.
Relays are cheap; many are shared across positions in the fuse box, making the swap-with-neighbor test free. This humble part strands more people than its price suggests.
Repair options and cost
Typical US independent-shop ranges, parts and labor. Dealers run higher; regional labor rates vary.
Owner reports mentioning P0685
“The contact owns a 2020 Ford F-250. The contact stated that while in drive(D), the accelerator pedal was depressed; however, the vehicle failed to accelerate as intended. The contact stated that the accelerator pedal was pumped gradually, and the vehicle accelerated as intended. The check engine warning light was illuminated. The vehicle was taken to an independent mechanic who retrieved DTC code: P0685. The contact was informed that the PCM relay needed to be replaced. The vehicle was repaired; however, the failure persisted. The manufacturer was notified of the failure. The failure mileage was 91,000.”2020 Ford F-250 SD · Oct 2023 · 91,000 mi
Cars that report P0685 most often
Based on mentions of this code in NHTSA owner complaints.
Frequently asked questions
Can I drive with P0685?
If it starts and stays running, yes to a shop - but no-start episodes are the signature.
How much does it cost to fix P0685?
Typical US repair costs range from $30 for the simplest fix (main power relay) to $300 in the worst case. The right number depends on which cause your car has - see the causes list.
Definition per SAE J2012 standard nomenclature. Symptoms, causes and costs are editorial guidance - always confirm with a diagnosis of your specific vehicle.