What P0686 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
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 energized 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.
Written from public technical sources and not yet checked by a mechanic. How we write these pages
Symptoms
- Check engine light
- Relay stays on after ignition off, battery drains
- Possibly P068B as well
Common causes
- Control wire between relay and module chafed to ground Most likely
- Relay shorted internally or water in the relay carrier Possible
- Module driver shorted internally Less likely
How to diagnose P0686
- 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
- 86
- Specific fault
Questions drivers ask
Can I drive with code P0686?
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 P0686?
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 P0686 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 P0686 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.