P0688
ECM/PCM Power Relay Sense Circuit/Open
Engine control module power
Generic OBD-II
High
What it means
P0688 means the engine computer isn't seeing the expected voltage on the sense wire that confirms its main power relay has switched on. Because this relay powers the ECM/PCM, an open in this circuit can cause hard-to-diagnose no-starts and stalling, so it's worth tracing the relay, fuse, and wiring carefully.
High
Can I keep driving? Risky — because this circuit powers the engine computer, the vehicle may crank but not start, or may stall unexpectedly. Get it diagnosed before relying on it.
Common symptoms
What you'll notice
- Engine cranks but won't start, or starts then stalls
- Intermittent no-start or stalling
- Check-engine light on
- Other seemingly unrelated codes stored when the module loses power
Common causes
Most likely first
- Failed ECM/PCM main power relayLikely
- Open or corroded wiring in the relay sense circuitCommon
- Blown fuse feeding the relay circuitCommon
- Poor ground or loose main module connectorLess often
- Failed ECM/PCM (least common)Less often
How to diagnose & fix
Work through it in order
- Scan for stored codes and check for a blown fuse in the relay/module power circuit.
- Test the ECM/PCM power relay — swap with a known-good identical relay to confirm.
- Check the sense wire between the relay and module for continuity (open circuit).
- Inspect grounds and the main module connector for corrosion or looseness.
- Replace the relay or repair the wiring; only suspect the ECM/PCM after power and grounds check out.
FAQ
Quick answers
Is it safe to drive with P0688?+
Not really. This circuit confirms power to the engine computer, so an open here can cause sudden stalling or a no-start. Have it diagnosed before depending on the vehicle.
Can I just clear P0688?+
You can, but if the relay, fuse, or sense wiring is faulty it will return — and may leave you stranded. Fix the power path first.
Related codes
Often seen with P0688
Read & clear P0688 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX