P0685ECM/PCM Power Relay Control Circuit/Open
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- 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. 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.
- What you may notice
- Check engine light
- Engine does not start or stalls
- No communication with the engine module (Saab)
- MIL flashes rapidly, other relays click (Saab)
- Likely causes
- LikelyOpen or corroded control wire between relay coil and module or connector
- LikelyRelay coil open or relay badly seated
- PossibleRelay coil supply missing (fuse, battery positive)
- RareWeak battery or voltage dip
- RareModule driver faulty
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Often possible to do yourself.Typical cost: €20 – €350
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.