What it means
The ECM or PCM power relay control circuit signal is higher than expected. The code alone does not identify the cause.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The ECM or PCM power relay supplies power to the engine controller, and the controller monitors the relay’s control circuit. P0687 sets when that control signal is higher than expected, which can happen if the wire is shorted to voltage or the relay or connector is faulty.
Common causes
Most likely first.
- Short to voltage in control circuit
- Faulty power relay
- Poor connector contact
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, voltage, and communication codes before clearing the fault.
- Check whether the controller stays powered or behaves inconsistently during key-on and shutdown.
- Inspect the relay socket and control harness for damaged insulation, contact with a power source, corrosion, or loose terminals.
- Measure the control-circuit signal and check for a short to voltage using the manufacturer’s wiring diagram and specifications.
- Test relay operation and controller power and ground before diagnosing a controller fault.
Common fixes
- Repair control wiring shorted to a voltage source.
- Repair loose, corroded, or damaged relay-socket connections.
- Replace a relay that does not pass the manufacturer’s test.
- Repair or replace the controller only after verifying the control circuit and relay.
Can I keep driving?
The fault can disrupt engine-controller power and may cause a no-start or stalling, so avoid driving if the vehicle behaves unpredictably. Have the relay circuit checked promptly, especially if other electrical or communication faults appear.
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.