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 status

Needs verification

Not yet verified against SAE J2012_202509 / J2012DA. Treat unverified rows as a lookup aid, not repair authority.

System: Computer
Scope: iso-sae-controlled-p0
Standard: SAE J2012 / ISO 15031-6
Source: legacy-import

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.

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 fault area. They may share the same verification status as the source database.