P0685 ECM/PCM Power Relay Control Circuit/Open

P0685 indicates an open or general fault in the ECM or PCM power relay control circuit. The controller cannot control or monitor its main relay path correctly.

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 condemn the PCM before proving the power relay, fuses, ignition feed, grounds, and load-tested outputs.

Useful checks before replacing parts

Check ignition feed, relay command, relay coil voltage, relay output, PCM power feeds, grounds, battery voltage, and freeze-frame or no-start symptoms.
Inspect the power relay, fuse block terminals, fusible links, ignition switch feed, ground control wiring, connector corrosion, and aftermarket alarm or remote-start wiring.
Load-test relay output and PCM power supply circuits to confirm whether the fault is relay control, relay contacts, wiring, or module driver related.

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.