What it means
The voltage sense line that confirms the main ECU power relay has actually closed reads low - either the relay is not closing or the sense line is shorted to ground.
At a glance
- Usually lights the check-engine light
How it works
The ECM or PCM power relay supplies operating power to the engine controller, and the sense line reports whether relay output is present. P0689 sets when that sensed voltage is lower than expected, which can mean the relay did not close or the sense line is shorted to ground.
Common causes
Most likely first.
- Relay does not close
- Connector pins corroded - high contact resistance
- Harness chafing through insulation
Symptoms
- Check engine light on
- Possible limp mode or no-start
- Multiple follow-up faults across modules
- Reset/relearn may be needed after repair
How to diagnose it
- Record freeze-frame data and scan for related relay, low-voltage, and communication codes to identify when relay output was lost.
- Check whether the engine controller wakes up consistently and whether symptoms appear during key-on or cranking.
- Inspect the relay and socket for damaged or corroded contacts, then examine the sense wire for chafing or contact with ground.
- Compare voltage at the relay output with the sense-line reading using the manufacturer’s wiring diagram and specifications.
- Test relay operation and check the sense circuit for a short to ground or excessive connection resistance before suspecting the controller.
Common fixes
- Repair chafed sense wiring or a short to ground.
- Clean or replace corroded relay-socket or sense-circuit terminals.
- Replace a relay that fails to close or has faulty contacts.
- Repair or replace the controller only after relay output and sense wiring are verified.
Can I keep driving?
The controller may not receive reliable power, which can cause a no-start or stall; avoid driving if either occurs. Address the fault promptly because intermittent power loss can create additional module faults.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.