P0688

ECM/PCM Power Relay Sense Circuit/Open

Q: 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.

Q: 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.

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.

Real-world scan data

Seen in

scans in the past year

Found in

<0.1%

of scans

<0.1%

of scans that found faults

Figures reflect only diagnostic scans recorded through our own apps and tools. Cars are typically scanned because a symptom or warning light prompted it, so these counts describe our scan records — not the incidence of faults in any model, and not the quality, reliability, or safety of any vehicle. No comparison between manufacturers or models is implied. All trademarks belong to their respective owners; no affiliation or endorsement is implied.

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.

Read & clear P0688 yourself

Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.

▶ Get it on Google Play

✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX