P0688: ECM/PCM Power Relay Sense Circuit/Open
The engine control unit has detected a fault with its main power relay or the relay circuit.
High risk
The engine may not start, stall or stay powered after switch-off. Have it checked soon.
Short answer · Can I keep driving with P0688?
With caution. A relay fault can stop the engine or keep it from starting, and on some cars it drains the battery because the control unit stays on. Have it checked soon.
What does P0688 mean?
P0688 means the engine control unit does not see the voltage the main power relay should deliver back to it. A failed relay, a blown fuse, a broken feed wire or a poor ground are typical causes. The main power relay supplies the engine control unit, and often the injectors, ignition coils and sensors, while the ignition is on and for a short time after it is switched off.
What the car does when the code is set
The check engine light comes on. Depending on the fault, the car may run normally, go into limp mode with limited power or stall. Other warning lights may appear because of the internal fault.
How the system works
The engine control unit runs continuous self-tests on its processor, memory and internal communication. Many units have a second, independent monitoring processor that checks the main one, especially for safety-relevant functions such as the electronic throttle. If these checks find a mismatch, a memory error or a reset, the unit stores a processor fault code and may switch to a limp-home mode with limited power to keep the car controllable.
Common symptoms
Possible causes
Listed without ranking. The cause on your car depends on make, engine and mileage — see the maker notes below.
- Faulty main power relayThe relay that supplies the engine control unit and other engine parts has failed or its contacts are burnt.
- Internal control unit faultThe processor or memory self-test fails.
- Supply or ground problemLow voltage or a poor ground can make the self-test fail.
- Water damageMoisture inside the unit.
- Software faultAn interrupted update or a known software issue.
How it is diagnosed
- Check supply and grounds firstMeasure voltage and grounds at the control unit as in the service data.
- Check for software updatesManufacturers sometimes release updates that fix this code.
- Inspect the unit and connectorLook for water ingress and corrosion.
- Repair or replace with codingA replacement control unit usually needs programming and immobiliser matching.
Questions about P0688
Can a weak battery set P0688?
Yes. Very low voltage while cranking or a poor battery ground can upset the relay circuit. Check the battery and its terminals before replacing the relay.
Can a jump start damage the control unit?
Voltage spikes from incorrect jump starting can damage electronics. Always follow the manual's procedure.
Is a used control unit an option?
Only if it can be programmed to your car, including immobiliser data. A specialist can advise.