P0686: low power-relay control voltage can overlap an open fault

P0686 is the low-voltage control-circuit fault for the powertrain relay. In the cited low-side driver implementation, the controller checks for a ground-short signature while the relay output driver is off.

Why off-state voltage matters

A low voltage while a low-side driver is intentionally active is not the defining condition here. The diagnostic needs the unexpected low reading during the off state. Its circuit-detection specification corresponds to less than 0.5 ohm from output to controller ground, with controller-specific voltage limits.

The monitor requires run/crank voltage above 11 volts and evaluates eight failures out of ten samples at 250 milliseconds per sample. Consequently, an observation made during a deeply depressed supply condition may not represent an enabled result.

Do not count companion codes as separate failures

The source states that P0685 can also set in certain controllers. That open-circuit companion can be part of an overlapping detection response; it does not alone establish a second damaged wire or relay. The actual architecture must guide interpretation.

Record the command state and paired codes, then follow the specific relay-control diagnostic. A successful repair verification should show the off-state electrical signature has normalized and the enabled monitor passes. This is a control-driver diagnostic, not a direct measurement of every circuit powered by the relay. The table does not justify automatically replacing the controller, energizing the relay externally, or bypassing its normal shutdown control.

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 418.