The WH power vent is usually controlled by / responds to 2 or 3 automatic switches – an air pressure switch, a gas pressure switch, and a blower temperature switch. In normal conditions (it’s a simple version of the process), whenever the temperature drops inside the water tank, the temperature switch activates the power vent motor (or the relay that switches the motor ON), as soon as the proper air pressure is established inside the flue/vent pipe, the gas pressure switch opens and starts the burner.
So… it could be a bad water temperature sensor, malfunctioning air pressure switch, or maybe a faulty relay that activates the water heater power vent
. Try to find a manual with an electric diagram of this particular model so you know exactly what controls the power vent.
Please let me know if you have any further questions.