| Sounds like a stuck relay. If this is a combination microwave, hostile territory for the DIY. Once apart, the fan should have one wire connected to the main bundle of white wires, and another going to a little box with 3 more wires. Likely the thing is bad and not much of a job to replace once you identify it. Identifying it and replacing it will be more of a challenge if it is on a circuit board. Could push it into being more bother than worth. I am sure the landfills are full of stuff only needing simple repairs. | 
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