This video demonstrates how to test an exhaust fan motor on a microwave. The exhaust fan vents fumes outside the home, but if the motor is defective, it may not vent properly.
Other commonly replaced parts for a microwave that is not working:
Line fuse
Main control board
Thermal fuse
Thermoprotector
All of the information provided in this part testing video is applicable to the following brands:
Amana, Frigidaire, GE, Kenmore, LG, Maytag, Panasonic, Samsung, Sharp, Whirlpool