P0685
Universal (All Makes) Vehicle (OBD-II)
Severity:What Does This Error Mean?
P0685 means the ECM/PCM power relay control circuit is open — the circuit that keeps the engine computer powered up has a problem. The ECM uses a relay to maintain its own power supply while the engine is running. When this relay circuit fails, the ECM may lose power unexpectedly, causing the engine to stall or not start. This is a serious code because the ECM controls everything in the engine management system. Immediate diagnosis is recommended — the car may stall without warning.
Affected Models
- All vehicles 1996+
- Common in GM vehicles
- Common in Ford vehicles
- Common in Chrysler, Dodge, and Jeep vehicles
- Common in vehicles with aging fuse boxes or relay corrosion
Common Causes
- Faulty ECM/PCM power relay that has failed or has burned contacts
- Open circuit (broken wire) in the relay control wiring
- Blown fuse for the ECM power relay circuit
- Corroded or loose ECM power relay connector
- Failed ECM self-hold circuit preventing the ECM from maintaining its own power
How to Fix It
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Locate the ECM power relay in the main fuse/relay box under the hood. Check the relay fuse first — a blown fuse is a quick and inexpensive fix.
Your owner's manual or fuse box diagram will identify the ECM main power relay location.
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Swap the ECM power relay with a matching relay from another circuit in the fuse box to test. If the code clears and the car runs normally, replace the original relay.
This is the fastest and cheapest test you can do — it costs nothing and takes under one minute.
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Inspect the relay connector and all wiring from the relay to the ECM. Look for damaged insulation, burned terminals, or any sign of overheating near the relay socket.
High current draw through a poor connection causes heat damage over time. Burned plastic near the relay is a red flag.
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Test the relay control circuit. With the relay out, turn the key on and check for voltage on the coil supply pin and a ground signal on the coil control pin. Both must be present for the relay to energize.
The ECM typically provides the ground to activate its own power relay — this is called a self-hold circuit.
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After repair, clear the code and start the engine. Monitor for stalling or unexpected shutdowns. If the engine stalls randomly, the repair was incomplete or the ECM itself was damaged.
Random stalling after this repair suggests the ECM may have sustained damage from the intermittent power loss.
When to Call a Professional
ECM power relay replacement is a straightforward DIY repair. Relays cost $10 to $25. If wiring is damaged, repair at a shop costs $100 to $300. If the ECM has been damaged by intermittent power loss, ECM replacement costs $200 to $600 or more. Do not drive far with this code — the car may stall at any time.
Frequently Asked Questions
Can P0685 cause sudden engine stalling?
Yes — this is one of the most serious symptoms. If the ECM loses power while driving, the engine will shut off immediately. This can happen without warning at any speed, which is a safety hazard. Do not drive far with this code active.
How do I find the ECM power relay?
Open the main fuse box under the hood. Look at the diagram on the lid or in the owner's manual for a relay labeled ECM, PCM, or Engine Control Module. On some vehicles it may be labeled Main Relay or Powertrain Control. If you cannot find it, your repair manual will show the exact location.
Can a bad ground cause P0685?
Yes. The ECM control circuit includes both a power supply and a ground path. A broken or high-resistance ground on the relay control circuit will prevent the relay from energizing. Always check ground connections as part of your diagnosis.