P0691: cooling-fan relay control is unexpectedly low while off

P0691 is the cooling-fan 1 relay control low-voltage fault in the cited output-driver monitor. It evaluates the low-side control circuit while the driver is off; low voltage in that state indicates the documented ground-short signature.

The monitor does not measure airflow

This is an electrical control-circuit diagnostic. It does not directly measure fan speed, air movement or engine cooling effectiveness. A fan that appears to turn cannot by itself prove that this driver monitor passes, and a stopped fan alone does not establish P0691's electrical condition.

The source requires powertrain relay voltage above 11 volts. The decision uses fifty failed samples out of sixty-three, at one hundred milliseconds per sample. The ground-short detection specification is less than 0.5 ohm between the signal and controller ground, using controller-specific voltage interpretation.

An open-circuit companion can occur

GM notes that P0480 may also set in certain controllers. That combination is not automatic proof of independent open and short failures. Preserve both codes and establish the actual driver architecture before interpreting their overlap.

A suitable verification checks the circuit in its relevant off state with adequate relay supply, then confirms the diagnostic passes using the vehicle-specific procedure. Keep clear of any fan that can be commanded during testing. The table does not identify a fan motor replacement, connector location or safe method for externally powering the circuit.

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 422.