P0688 means ECM/PCM Power Relay Sense Circuit/Open. The engine computer is not seeing the expected voltage on the circuit that confirms its main power relay has switched on. It is a generic OBD-II code, so the definition is the same on any make, but the most likely cause varies by engine; confirm with a scan tool and the service information for your vehicle before buying parts.
What P0688 means
Modern vehicles have many control modules that share information over a network, usually called the CAN bus. Codes starting with U mean a module has stopped hearing another one, and voltage and processor codes mean a module is not getting clean power or has an internal fault. Low battery voltage, poor grounds, blown fuses and damaged wiring cause many of these codes, so test the battery and charging system first.
P0688 symptoms
- No start or intermittent no-start
- Stalling
- Battery drain if the relay sticks on
- Check engine light
Common causes of P0688
- Failed main power relay
- Blown fuse in the relay feed
- Open wiring or a corroded connector
How to diagnose P0688
- Check battery voltage and terminals
- Test or swap the main power relay
- Check the relay’s fuses and the sense circuit to the computer
Can you drive with P0688?
Risky; the engine may stall or fail to start.
P0688 tips
Battery terminal corrosion can cause power relay codes; clean the terminals and check grounds first.
Will P0688 clear itself?
If the fault stops, the computer turns off the check engine light by itself after several drive cycles without seeing it again, and the code stays in memory as history for a while. Clearing P0688 with a scan tool after a repair is fine, but if the cause is still there the code returns, sometimes within one drive. Disconnecting the battery also clears codes on most vehicles, but it erases learned fuel trims and idle settings as well.
Related codes
| Code | Meaning |
|---|---|
| P0685 | ECM/PCM Power Relay Control Circuit/Open |
| P0686 | ECM/PCM Power Relay Control Circuit Low |
| P0687 | ECM/PCM Power Relay Control Circuit High |
Guides that help
When it is more than a sensor
Voltage and network codes come from wiring, the battery, the charging system or a module, not from engine or transmission wear. If a lost-communication code has left the transmission in limp mode, rule out the module and wiring before deciding the unit has failed, and see our transmissions only if testing confirms internal damage.
Shipping is a flat $80.00 per order to US addresses, with 15-day free returns. Questions? Ask a parts specialist.