What Would Cause a Microwave To Stop Working?

You’re just about to stream your favorite movie, but first you set the package of microwave popcorn on 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 end 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 common 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 the fuse will need to be replaced with a new one before the microwave will start working again. The main fuse is not the only fuse found in microwave ovens. There can also be thermal fuses, cavity fuses, and thermoprotectors which will interrupt the electrical flow if the microwave overheats. All of these parts can be tested with a multimeter to confirm continuity – a continuous electrical path present in the component:

  • Before you begin testing, unplug the microwave oven power cord.
  • Since high voltage capacitors used in microwaves may retain a charge even after the power cord has been unplugged, it is recommended to discharge the capacitor before servicing. You can do this 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 lead to 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; if the meter display shows no significant change, the component lacks continuity which means it has blown and will need 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 once the door is fully closed. A faulty door switch will prevent the microwave from working even after 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, and 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 lead to 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 has no continuity and is faulty.
  • For three-terminal switches, touch the black lead to the “common” terminal and the red lead to the “normally closed” terminal. The multimeter display should show zero Ohms of resistance. Move the red lead to the “normally open” terminal and the multimeter should indicate continuity only after the switch has been actuated.
  • Be aware, if either a two-terminal or three-terminal door switch tests positive for continuity when it should not, it is likely the switch has shorted closed.

Could the main control board be defective?

Although it’s not a common cause, a defective main control board could prevent the microwave from working as well. Since it’s difficult to test a control board accurately, you should confirm that the fuses, thermoprotector, and door switches all have 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 right microwave oven parts with Repair Clinic

Repair Clinic stocks all the replacement appliance parts that match your microwave oven, including fuses, door switches, and main control boards. Enter the full model number of your microwave in 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 need. While Repair Clinic carries parts that fit 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 microwave oven model.

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