P0685: powertrain relay open-circuit detection uses the off state

P0685 is an open-circuit fault in the powertrain relay driver in this GM example. The output is powered off while the controller compares its circuit voltage against a controller-specific acceptable range.

Control circuit versus delivered power

The monitor concerns the relay-driving circuit. It is not the separate feedback-voltage check that asks whether relay power actually appeared at a sense input. A vehicle may have related symptoms, but the code does not by itself establish which relay contact or downstream load failed.

The open-circuit detection specification is greater than 200 kilohms between output and controller ground. Run/crank voltage must exceed 11 volts, and the decision uses eight failures in ten samples at 250 milliseconds per sample. An off-state check performed below the enabling voltage does not establish a passing diagnostic.

Companion codes can share a signature

GM explicitly notes that P0686 may also set in certain controllers. Therefore, open-circuit and low-circuit codes appearing together do not automatically prove two separate defects. The controller architecture and actual circuit evidence must resolve that ambiguity.

Preserve the commanded output state, voltage context and companion-code pattern before choosing the appropriate service test. Verification should include a passing off-state driver monitor and the vehicle's required power-control checks. The impedance value describes diagnostic capability, not a safe live-circuit test method. No universal relay replacement or bypass procedure is supported by this summary.

Source and applicability

Documented monitors in gm-2025-03a, model year 2025. These are module/calibration examples; vehicle, engine and transmission applicability must be confirmed independently. GM 2025-03a, page 417.