What P0685 means
The engine module switches its main relay coil to ground (control circuit), reads the switched voltage back on a sense line and keeps the relay on briefly after ignition off to save learned values.
How the engine computer detects it
P0685 means the relay control circuit is open: GM measures with the driver off and flags 200 kΩ or more to ground. GM checks with run/crank voltage above 11 V and sets the code after 8 of 10 samples at 250 ms, with the lamp after two trips. GM Europe's 2.0 L turbo compares commanded and actual state for 2 s, Ford's 2023 diesel checks continuously within 0.5 s. A real open keeps the relay from pulling in – no start; on Saab's V6 there is then no communication with the engine module and the MIL flashes. GM notes P0686 may set alongside.
Written from public technical sources and not yet checked by a mechanic. How we write these pages
Symptoms
- Check engine light
- Engine does not start or stalls
- No communication with the engine module (Saab)
- MIL flashes rapidly, other relays click (Saab)
Common causes
- Open or corroded control wire between relay coil and module or connector Most likely
- Relay coil open or relay badly seated Most likely
- Relay coil supply missing (fuse, battery positive) Possible
- Weak battery or voltage dip Less likely
- Module driver faulty Less likely
How to diagnose P0685
- Read every stored code with an OBD-II scanner. If misfire or fuel trim codes are there too, fix those first: they often cause this one.
- Inspect the parts involved: ecm power relay, relay control circuit, engine control module.
- Work through the causes above from most to least likely, cheapest checks first.
- Clear the code and drive a full drive cycle to confirm the repair held.
Reading the code
- P
- Powertrain
- 0
- Generic (SAE) code
- 6
- Computer and output circuits
- 85
- Specific fault
Questions drivers ask
Can I drive with code P0685?
Usually yes for a short time, if the car drives normally, the temperature gauge reads normal and the check engine light is steady. If the light flashes, the engine loses power or overheats, or other warning lights come on, pull over and have it checked.
Will my car pass an emissions inspection with P0685?
Probably not in states that test vehicles with an OBD-II scan: an emissions-related code that keeps the check engine light on is usually an automatic fail. Rules differ by state and county, so check yours.
Will P0685 clear by itself?
The light can go off after several drive cycles without the fault, but the code returns if the cause is still there. After a repair, the readiness monitors need a full drive cycle before an inspection.
Can I fix P0685 myself?
Often yes, with an OBD-II scanner and basic tools. Difficulty: medium. Start with the cheapest checks before replacing expensive parts.
This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.