P0686ECM/PCM Power Relay Control Circuit Low
- 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. P0686 means the control line is shorted to ground: with the driver off it should carry battery voltage through the coil, but it sits at ground (0.5 Ω or less). The relay then stays energised and the module can no longer switch it off – after-run and shutdown are disturbed and the battery can drain. 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. The former obdex title (the wording of P0685) was a shifted entry; P0686 is the 'low' code.
- What you may notice
- Check engine light
- Relay stays on after ignition off, battery drains
- Possibly P068B as well
- Likely causes
- LikelyControl wire between relay and module chafed to ground
- PossibleRelay shorted internally or water in the relay carrier
- RareModule driver shorted internally
- 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.