P0686
ECM/PCM Power Relay Control Circuit Low
Power supply / ECM relay
Generic OBD-II
High
What it means
P0686 means the engine computer detects a low-voltage fault in the control circuit for its own main power relay. Because this relay feeds power to the ECM/PCM and related systems, a fault here can cause hard starting, a no-start, or sudden stalling along with scattered electrical codes. It is worth treating as urgent.
High
Can I keep driving? Risky — this circuit powers the engine computer, so it can cause a no-start, sudden stalling, or intermittent electrical faults. Address it promptly.
Common symptoms
What you'll notice
- Check-engine light on, sometimes with other electrical warning lights
- Hard starting or a no-start condition
- Sudden or intermittent stalling
- Multiple unrelated sensor codes appearing together
Common causes
Most likely first
- Faulty ECM/PCM main power relayLikely
- Short to ground or open in the relay control wiringCommon
- Blown fuse or corroded connector in the power supply circuitCommon
- Poor ground or, less often, an internal ECM/PCM driver faultLess often
How to diagnose & fix
Work through it in order
- Scan for all stored codes and check the relay and its fuse first.
- Test the relay control circuit for a short to ground or an open.
- Inspect connectors, grounds, and power feeds for corrosion or looseness.
- Replace the relay if it fails testing; treat ECM/PCM replacement as a last resort after wiring is ruled out. Clear and re-test.
FAQ
Quick answers
Is it safe to drive with P0686?+
No — because it involves the engine computer's power supply, it can leave you stranded with a no-start or stall. Get it fixed promptly.
Can I just clear P0686?+
It will return, and the underlying relay or wiring fault can still cause a stall or no-start.
Related codes
Often seen with P0686
Read & clear P0686 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