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

Symptoms of P0685

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 P0685

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 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 pin down the fault area.