To repair a built-in microwave, you must first identify the actual problem. Common problems with built-in microwaves include faulty power connections, malfunctioning motors and defective magnetron tubes.
If a built-in microwave does not start, first check to be certain that the appliance is turned on and that the power cord is fully plugged into a working electrical outlet. Check whether the fuse has blown or the circuit breaker has tripped. Reset tripped circuit breakers and replace blown fuses. If the problem persists, confirm that the microwave door is closed properly and its latch is aligned.
If a built-in microwave does not heat, the magnetron tube may have burnt out. A magnetron uses high voltage current to produce the microwave frequency used to cook food. If it has burnt out, it should be replaced. Examine the high voltage diode and door switch. Use an Ohmmeter to test the continuity of these parts. If either of the parts is faulty, replace it accordingly.
If the microwave is noisy or loud, the exhaust fan motor or turntable motor may be defective. Replace a faulty fan motor or turntable motor with a new one of the same kind. If the appliance is noisy when cooking, the magnetron tube might be defective and should be replaced.