Bad fan motor. Uses a "shaded pole" design. The rotor rests on an irregularity on the motor bearing, and the motor lacks the starting torque to initiate rotor motion. Sometimes works, sometimes not. Worse in cold when the little bit of lubrication in motor bearing turns to glue. Replace the fan motor. Testing will reveal the motor is OK 9 times out of 10 because removing the motor invariably turns the shaft. Radiator electric fans can also have the same problem.
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