P0685 Code: ECM Power Relay Circuit — Causes & Fix
P0685 means a fault in the ECM/PCM power relay control circuit — the main relay that supplies power to the engine computer.
- Applies to
- All OBD-II vehicles (any make or model)
- Est. repair cost
- $30 – $500
- Time to fix
- 1–4 hours
What it means
Causes include a failed main relay, wiring/connector faults, or a poor ground. P0685 is a generic OBD-II code defined in the SAE J2012 standard, so it applies to virtually any 1996-or-newer OBD-II vehicle regardless of make. Fix the underlying fault, then clear the code with an OBD-II scan tool — it will return if the problem isn't resolved.
Symptoms
- Check engine light on
- Intermittent stalling or no-start
- Electrical dropouts
How to fix it
- 1
Test the main relay and wiring
1. Check the ECM power (main) relay and its control/feed wiring.
- 2
Replace the relay
1. Replace a failed relay and repair wiring, then clear the code.
Frequently asked questions
Can P0685 cause a no-start?
Yes — if the main relay won't power the ECM reliably, the engine can stall or fail to start. Test the relay and its wiring.
Last updated July 19, 2026
Guidance only — always consult a qualified professional or the official service manual before carrying out repairs.