Four parts fail more than any others on microwave ovens:
Three additional parts are also common culprits, but not so often as the above: fuses, fan motors and a tray that fails to rotate.
All these repairs involve diagnosing the trouble spot, carefully removing the old part and replacing it with a suitable working part.
For such as small footprint in most people's kitchens, a microwave oven is dense with parts. The interconnected systems depend on signals from the control panel, but since they generate potentially dangerous microwave radiation, microwave ovens include safety switches on door latches that shut down the microwave if the door is forced open during operation. Homeowners do not usually notice the particular part that stops working, they notice that the microwave either does not operate at all or does not heat food properly.