Diagnosis
The touchpad for the vent fan is likely stuck or shorted. Moisture or grease can cause a phantom press.
How to Fix It
Clean the entire control panel thoroughly with a damp cloth, focusing on the vent control area. Let it dry completely. Press the vent OFF button firmly several times.
If That Didn't Work
The control board may have a failed relay. This requires a main PCB replacement. Unplug the unit to stop the fan until it's repaired.
Heads Up
A constantly running fan draws power and wears the motor. Do not leave it in this state for extended periods.