What it means
The ECM or PCM power relay control circuit signal is lower than expected. The code alone does not establish whether the fault is in the relay, wiring, or controller.
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 operating power to the engine controller, which monitors the relay’s control circuit. P0686 sets when the control signal is lower than expected, such as from a short to ground, a weak connection, or a relay problem.
Common causes
Most likely first.
- Short to ground or open 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 to identify whether the fault coincides with a start or shutdown event.
- Check for intermittent no-start, stalling, or controller resets and note whether the fault returns at key-on.
- Inspect the relay, socket, and control wiring for corrosion, loose terminals, chafing, or contact with ground.
- Measure the control-circuit signal and check for a short to ground or an open using the manufacturer’s wiring diagram and specifications.
- Test the relay and verify controller power and ground under load before considering an internal controller fault.
Common fixes
- Repair wiring shorted to ground or an open in the control circuit.
- Clean or repair poor relay and harness connections.
- Replace a relay that fails the specified test.
- Replace or repair the controller only after the relay circuit and its power and ground are confirmed sound.
Can I keep driving?
Because the engine controller may lose power, the vehicle could stall or fail to start; avoid driving if symptoms occur. Arrange prompt repair, and do not rely on it for a journey where a sudden shutdown would be hazardous.
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.