Micorwave ovens - and where's the heat?

A microwave not heating up is caused by one or more of the following -

1) Failed controller board (unlikely if you are getting a clock readout and you can get the various functions to work, just no heat)
2) Failed door switch
3) Failed thermal fuse, when used
4) Blown power fuse that is dedicated to the microwave circuit
5) Bad diode
6) Bad capacitor
7) Bad magnetron
8) Bad transformer

Door switches insure the microwave circuit will not engage when the door is open. Slamming the door of the microwave is hard on the switch.

The thermal due is wired into the microwave harness, and is a melting allow type, based on heat buildup within the oven. Always replace with a fuse of the same thermal rating.

The power fuse is a special type and should only be replaced with that same type and rating. It is easily tested.

The diode is a relatively inexpensive part, and is not easily tested. If a grumble is heard coming from electrical area of the unit, then the diode could be shorted.

A shorted capacitor will blow the fuse immediately. And open capacitor will cause a "no heat" condition.

A bad magnetron will also cause a low level hum or grumble from the transformer. But is may also cause no other identifiable symptom of failure.

A bad transformer will exhibit a grumble or hum, and may or may not blow the fuse. But be careful here, the secondary of the transformer produces very high voltages, up to 2.5Kv. Few people own a meter capable of measuring such voltages.

Service and repair of microwave ovens is dangerous work. If you don't know what your doing, DO NOT ATTEMPT TO SERVICE A UNIT WITHOUT INSTRUCTION.