This video demonstrates how to test a three terminal switch on a microwave. The door switch is a commonly replaced part for a microwave that is not working.
Other commonly replaced parts for a microwave that is not working:
Main control board
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