What P0688 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
P0688 means the expected switched voltage does not arrive on the sense line – the line is open, a fuse in the monitored path has blown or the contact does not close. Chrysler (Stellantis) calls the relay 'auto shutdown (ASD) relay'; a service case for intermittent no-starts with P0688 points first to spread or pushed-back terminals in the ASD circuit connector. GM does not use P0688 and reports an open sense line as P0689. The former obdex title (a duplicate of P0690) was a shifted entry.
Written from public technical sources and not yet checked by a mechanic. How we write these pages
Symptoms
- Check engine light
- Engine cranks but intermittently does not start (Stellantis)
- Engine stalls while driving
Common causes
- Terminal in the sense or ASD circuit connector bent, pushed back or spread Most likely
- Blown fuse in the monitored load circuit (e.g. shorted ignition coil) Possible
- Relay contact does not close Possible
- Module input faulty Less likely
How to diagnose P0688
- 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.
Vehicles that report P0688 most
| Vehicle | Years | Reports | Share | Median miles |
|---|---|---|---|---|
| Jeep Grand Cherokee | 2007–2008 | 4 | 64k |
Counts of owner complaints to the NHTSA that mention P0688 (6 in total), not failure rates: popular models get more complaints simply because more of them are on the road. Models with fewer than 3 complaints are left out.
Reading the code
- P
- Powertrain
- 0
- Generic (SAE) code
- 6
- Computer and output circuits
- 88
- Specific fault
Questions drivers ask
Can I drive with code P0688?
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 P0688?
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 P0688 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 P0688 myself?
Often yes, with an OBD-II scanner and basic tools. Difficulty: medium. Start with the cheapest checks before replacing expensive parts.
Often reported together
Codes that owners mention in the same NHTSA complaints as P0688. They can point to a shared cause, so it's worth checking them together.
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.