P0688: Engine Control Module Power Relay Sense Circuit Open
What to do
May or may not start. Worth measuring whether the relay is actually switching before it is replaced — the fault may be only in the confirmation circuit, in which case the relay is fine.
What this code means
An open on the circuit through which the module confirms the power relay actually closed. The sense circuit is the feedback half of the arrangement — the module commands the relay and then checks that voltage appeared where it should.
A sense fault with the relay working produces a code and little else. A sense fault because the relay genuinely did not close produces a no-start. Measuring for voltage at the relay's output separates the two immediately.
Likely causes
Start here
- Failed ECM power relay · easy
- Broken wire in control circuit · moderate
- Corroded relay socket · easy
Less common
- Blown actuator supply fuse · easy
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
Be the first to write this up
Nothing posted about this yet. Whatever you found is worth writing down — even a dead end saves someone the same afternoon.
What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
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