P0685
ECM/PCM Power Relay Control Circuit/Open
ECM/PCM power supply
Generic OBD-II
High
What it means
P0685 means the control circuit for the relay that powers the engine/powertrain control module has an open or fault. Because this relay feeds the computer that runs the engine, an unreliable connection can cause hard starting, sudden stalling, or a complete no-start. The usual cause is a failed power relay or damaged wiring in its control circuit.
High
Can I keep driving? Often not — an unreliable power feed to the engine computer can cause hard starts, sudden stalling, or a no-start, so treat it as urgent.
Common symptoms
What you'll notice
- Engine won't start or is hard to start
- Sudden stalling or the engine cutting out while driving
- Check-engine light on
- Intermittent electrical glitches or dashboard warnings
Common causes
Most likely first
- Failed ECM/PCM power relay (most common)Likely
- Open or damaged wiring in the relay control circuitCommon
- Loose or corroded connectors or poor groundsCommon
- Faulty ECM/PCM (rare)Less often
How to diagnose & fix
Work through it in order
- Locate the ECM/PCM power relay and test it; swap in a known-good relay if in doubt.
- Check the relay control circuit for continuity and the expected switching signal from the ECM.
- Inspect connectors, fuses, and ground points for corrosion or looseness.
- Repair damaged wiring, then retest for a stable power feed.
- Only replace the ECM/PCM after the relay, wiring, and grounds test good. Clear and re-test.
FAQ
Quick answers
Is it safe to drive with P0685?+
Not really. If the computer loses power the engine can stall without warning — get it diagnosed before relying on the car.
Can I just clear P0685?+
Clearing it won't help; the fault returns whenever the relay circuit misbehaves, and the underlying relay or wiring must be fixed.
Related codes
Often seen with P0685
Read & clear P0685 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX