If the microwave does not heat, one of the door switches might be defective. Microwave ovens normally have three door switches, if any of them fail the microwave does not turn on and does not heat. Check the switches for continuity with an Ohm meter.
If the microwave oven doesn't heat the magnetron tube might have burned out. The magnetron uses high voltage, high current DC power to generate the microwave frequency that cooks the food. If the microwave oven is turned on when it is empty this can cause the magnetron to burn out. Once it is burned out it has to be replaced, it's not repairable.
High Voltage Capacitor
If the microwave doesn't heat the high voltage capacitor might be defective. The high voltage capacitor works with the high voltage diode to convert the output of the transformer to DC voltage and to double the output voltage. If the capacitor is burned out the entire high voltage circuit stops working properly. The high voltage capacitor can be checked with a special VOM meter which has a capacitance testing capability. Be aware that the high voltage capacitor can retain a charge of more than 3,000 volts and can injure or kill a person if not handled properly. Only trained technicians should perform this type of testing.
High Voltage Transformer
If the microwave does not heat, the high voltage transformer might be burned out. Microwave ovens produce a very high voltage in order to power the magnetron antenna, which emits the energy that cooks the food. When a high voltage transformer fails it will usually arch and have a burning smell.
If the microwave doesn't heat the thermal fuse may have blown. It can be tested for continuity. Watch our fuse testing video for more information.
If the microwave doesn't heat the thermoprotector may have tripped. This is a safety device to prevent the microwave from overheating. It can be tested for continuity to see if it's blown.
Main Control Board
If the microwave doesn't heat the main control board may be defective. This is not common. When a microwave doesn't heat the problem is usually a faulty door switch or within the high voltage circuit; either the high voltage capacitor, diode, transformer or magnetron.