My answer to Why is my microwave not working?
Answer by KC Yee:
most common failure in microwave ovens is the electrical relay that is controlled by the control panel. The relay is a electric-mechanical device, that is no more than a controlled switch, it uses a metal piece for connection, and a spring to bounce off the metal to disconnect. This is often where it fails.
the part only costs a few $, but Unfortunately, this part is NOT user replaceable. Requires a professional service person.