Engine Control Module (ECM) Power Relay Sense Circuit/Open
The ECM power relay circuit has detected an issue.
Heads up: this code wasn't in our hand-written library yet, so DiagCoach AI built this page on the fly. Double-check critical specs against service info before condemning parts.
What it means (plain English)
The Engine Control Module relies on a stable power source to function, and this code sends a signal when that power supply is interrupted or faulty.
What the computer is actually seeing
The ECM monitors the power relay circuit for expected voltage levels. When it senses an open circuit (no voltage) or a voltage level outside of the typical range, it sets the P0685 code.
What a healthy reading looks like
The ECM relay should have a nominal voltage around 12V when energized. If the voltage drops below 9V or is completely absent, this error code may trigger.
Guided diagnostic — the DiagCoach way
Don't just throw parts at it. Walk through these in order — each step tells you whether to keep going or stop and fix what you found.
- 11. Check the battery voltage; it should be between 12.4V and 12.7V with the engine off. If low, charge or replace the battery.
- 22. Inspect the power relay for signs of damage or overheating; swap with a similar relay if available to test.
- 33. Examine the wiring and connectors between the relay and ECM for any visible damage, corrosion, or loose connections. Repair or replace as needed.
- 44. Measure voltage at the ECM power relay connector with the key on (engine off); it should read around 12V. If no voltage, trace back to the ignition switch and battery.
- 55. If steps 1-4 check out but the code remains, there may be an internal fault in the ECM. Consider bench testing or replacement.
Common causes
- Faulty ECM power relay
- Damaged wiring or connectors in the relay circuit
- Weak battery or poor electrical connections
- Defective Engine Control Module (ECM)
- Corrosion or short circuit in power supply circuit
Typical repair cost
$$
Related codes
Frequently asked questions
Is it safe to drive with this code?
It's generally safe for short distances, but address it soon to avoid potential starting issues.
Will this code come back after clearing?
If the underlying issue isn't fixed, it likely will return after clearing.
Why does this code sometimes appear with other codes?
It's common for multiple codes to show if there's an electrical issue affecting the ECM and related systems.
Working a real vehicle right now?
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