P0685 — Power relay circuit for engine control module open or faulty

The engine control module (ECM) power relay circuit is not working properly or has an open connection. This relay supplies power to the ECM/PCM, and if it fails, the engine may not start or run correctly. The code appears when the control module detects no power relay activation or an electrical fault in the relay circuit.

Common causes

The cause is almost always one of a few things. Here they are — P0685 rarely comes from anything outside this list:

• Faulty ECM/PCM power relay
• Blown fuse in relay circuit
• Damaged wiring or connectors to relay
• Corroded or loose relay socket terminals
• Faulty ECM/PCM control output

Where to start

In the order a sensible mechanic would work — the cheap checks first, the expensive replacements only after:

1. Check and replace blown fuses in relay circuit
2. Inspect and repair wiring and connectors
3. Test and replace faulty power relay
4. Clean or replace corroded relay socket
5. Diagnose ECM/PCM output if relay and wiring are good

That is what holds for P0685 in general. From here it depends on your engine — tell me which car it is and what it does, and I will give you the steps for it: real procedures, torque figures and diagrams instead of general advice.