P0688 ECM/PCM Power Relay Sense Circuit/Open

P0688 indicates an open or general fault in the ECM or PCM power relay sense circuit. The controller cannot verify that the main relay output is present.

Code details

Generic OBD-II · P0

System: Computer
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not replace the relay only because the code mentions relay sense; verify the sensed voltage at the controller.

Symptoms of P0688

Check-engine light, sometimes with several unrelated codes at once.
Warning lights, a no-start, or intermittent electrical faults.
Communication or module-reset problems on a scan tool.

Common causes of P0688

Low battery voltage, poor grounds, corroded connectors, or blown fuses.
Wiring or network (CAN bus) faults between modules.
A module software/programming issue, or rarely a failed controller.

Useful checks before replacing parts

Check power relay sense voltage, relay output, PCM battery feeds, ignition feed, grounds, freeze-frame data, and related no-start or stall symptoms.
Inspect relay contacts, fuse box terminals, sense wire continuity, connector corrosion, fusible links, harness damage, and module power supply pins.
Load-test the relay output and sense circuit to confirm whether power is missing at the relay, lost in wiring, or not read by the module.

Related OBD-II codes

These nearby computer entries can help compare wording and pin down the fault area.