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Problem is going to be a bad fan control board or bad controller built onto end of motor.Do you have a multi meter for troubleshooting?
OK looks like the unit has a motor that is high voltage powered but low voltage controlled. Often the contacts in the bell on end of motor will stick.
Locate the low voltage wires from board Brown COM and the wire from LOW terminal on board. Disconnect those 2 wires and see if fan stops.
Yes there are contacts in the controller on motor itself for speeds controlled by low voltage.
If we disconnect the low voltage com and speed and motor still runs, we know issue is in that controller on motor. If not lets find where we might be getting 24 volts to that LOW terminal on board.Possibly a sequencer or something in heat kit.