The 3 most common part(s) or condition(s) which cause the symptom Microwave exhaust fan not working are listed below starting with the most likely. Check or test each item and watch any available videos.
RepairClinic Item # 1394041
If the exhaust fan does not run the fan motor might be bad. Remove the fan motor from the microwave oven and see if the blades can be turned by hand. They should turn very freely, if they're stiff the fan motor will need to be replaced. The fan motor can also burn out. To test this use an Ohm meter. The motor should have continuity. If it doesn't, replace it.
RepairClinic Item # 903788
If the microwave exhaust fan is not working the thermostat might be defective. The thermostat senses the air temperature and turns on the fan as needed, if it's defective it won't turn on the fan. Watch our video on testing thermostats.