You're just about to stream your favorite movie, but first you set the package of microwave popcorn around the microwave's glass turntable. You press the “Popcorn” button and- nothing. Or, you just want to heat up last night's leftovers for a quick lunch, but you wind up eating the lasagna cold. The microwave oven seemed to be operating fine a few days ago, so why did it suddenly stop working?
How to check a microwave fuse
The most typical cause for a microwave oven not working at all is a blown main fuse. The microwave main fuse will cut the flow of electricity if too much current passes through it. When this happens, the fuse is considered “blown” and also the fuse will need to be replaced with a new one before the microwave will start working again. The primary fuse is not the only fuse found in microwave ovens. There can also be thermal fuses, cavity fuses, and thermoprotectors that will interrupt the electrical flow when the microwave overheats. All of these parts could be tested with a multimeter to verify continuity – a continuous electrical path contained in the component:
- Before you begin testing, unplug the microwave oven power cord.
- Since high voltage capacitors used in microwaves may retain electric power charge even after the power cord has been unplugged, it is recommended to discharge the capacitor before servicing. This can be done by placing a screwdriver blade across each set of capacitor terminals (avoid touching the blade when doing this).
- Remove the fuse or thermoprotector from the appliance.
- Rotate the multimeter dial to the lowest setting for “Ohms of resistance”.
- Touch the black result in one of the component's terminals and the red lead to the other terminal.
- If the meter display shows zero Ohms of resistance, the component has electrical continuity; when the meter display shows no significant change, the component lacks continuity meaning it has blown and will have to be replaced.
Keep in mind that a blown fuse is often caused by a faulty door switch.
A faulty door switch can prevent microwave from working
Most microwaves have three or four door switches that allow the appliance to start or heat when the door is fully closed. A faulty door switch will prevent the microwave from working despite the door has been closed. As with the fuses and thermoprotector, you can test each door switch for continuity to determine if one has failed:
- Two-terminal door switches will either have continuity before the switch is actuated or after; three-terminal door switches will have a “common” terminal, a “normally closed” terminal that provides continuity before the switch is actuated, along with a “normally open” terminal that provides continuity after the switch is actuated.
- For two-terminal switches, touch the black lead to one terminal and the red result in the other terminal, then actuate the switch. If the meter display shows zero Ohms of resistance, the switch has continuity; if the meter display shows no significant change, the switch doesn't have continuity and is faulty.
- For three-terminal switches, touch the black result in the “common” terminal and the red result in the “normally closed” terminal. The multimeter display should show zero Ohms of resistance. Move the red lead to the “normally open” terminal and also the multimeter should indicate continuity only after the switch has been actuated.
- Be aware, if whether two-terminal or three-terminal door switch tests positive for continuity if this should not, it is likely the switch has shorted closed.
Could the primary control board be defective?
Although it isn't a common cause, a defective main control board could avoid the microwave from working as well. Since it's difficult to test a control board accurately, you need to confirm that the fuses, thermoprotector, and door switches have the ability to electrical continuity and are functioning normally before you consider replacing the board.
Safety Warning
Because of the high voltage and high current used by microwave ovens, be aware that repairing this appliance poses a substantial risk for injury or death if precautions are not taken. You should always unplug the microwave before you attempt any disassembly. Since high voltage capacitors used in microwaves may retain a charge even after the microwave has been unplugged, we recommend that only experienced professionals access and replace internal components.
Find the best microwave oven parts with Repair Clinic
Repair Clinic stocks all of the replacement appliance parts that suit your microwave oven, including fuses, door switches, and main control boards. Go into the full model number of your microwave within the Repair Clinic website search bar to see a complete list of compatible parts, then use the part category and part title filters to narrow the list down to identify the exact part you'll need. While Repair Clinic carries parts that suit microwaves from top brands like GE, Samsung, Whirlpool, LG, Kenmore, Frigidaire, and Panasonic, you'll want to make sure you're selecting a part that is directly associated with your specific micro wave model.
