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.

Real-world scan data

Seen in

3,442

scans in the past year

Found in

0.5%

of scans

1.0%

of scans that found faults

Appears alongside · share of P0685 scans

P0683in 22.6% of scans

P0110in 20.7% of scans

P0480in 19.7% of scans

Figures reflect only diagnostic scans recorded through our own apps and tools. Cars are typically scanned because a symptom or warning light prompted it, so these counts describe our scan records — not the incidence of faults in any model, and not the quality, reliability, or safety of any vehicle. No comparison between manufacturers or models is implied. All trademarks belong to their respective owners; no affiliation or endorsement is implied.

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.

Read & clear P0685 yourself

Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.

▶ Get it on Google Play

✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX