Why Did My Microwave Vent Fan Turn on All by Itself?

Do you have a microwave vent fan that turns on by itself? Before you blame mischievous ghosts, know that a technical issue is likely the problem. Issues with your microwave oven or the fan itself can trigger an automatic exhaust fan. Heat and humidity often play a strong role.

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A professional can help you pinpoint and resolve these concerns, whether your microwave fan runs constantly or only occasionally turns on by itself. Keep reading to learn how to stop this unexpected phenomenon.

Why Did My Microwave's Exhaust Fan Turn on by Itself?

You're relaxing on the sofa, about to watch your favorite show, when you hear the fan kick on in the kitchen. It can be alarming when a microwave vent fan turns on by itself, but don't panic. The cause may be as minor as grease buildup or your fan needing an upgrade.

Let's review some common reasons a microwave vent fan turns on by itself.

Grease or Dust Buildup

When was the last time you cleaned the vent filters under your microwave? If you have no idea — or didn't realize your microwave had filters — they're likely overdue for cleaning.

When you cook on your stovetop, the fumes transport dust, grease and other debris into the air. Your filters catch this debris as long as you keep them clean. Clogged filters can block airflow, causing your fan to work harder than usual. Keep your filters clean by washing or replacing them at least once every few months, more often if necessary.

System Overheating

Many over-the-stove microwaves have built-in vent fans that kick on automatically if the kitchen gets too hot. This feature helps prevent your microwave from overheating, but it shouldn't kick on for no reason.

If your microwave fan runs constantly, you may need a new thermostat or heat sensor. However, you must first determine whether your microwave is actually overheating or your sensor is detecting a nonexistent problem. A professional can help you decide.

Faulty Temperature Sensor

Your thermistor, commonly called a temperature sensor, lets your microwave know when things get too hot. When the sensor malfunctions, it may determine your kitchen is hotter than it actually is. This can result in a microwave vent fan that turns on by itself.

Electrical Issues

Electrical issues can cause a microwave exhaust fan to work improperly. Your microwave fan may be loud, run constantly or turn on by itself. That's because the wires are sending the wrong signals to each other. This isn't a DIY fix, so call an electrician for help.

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What Should I Do If the Vent Fan Keeps Coming on When I'm Not Using the Oven or Stove?

If you're tired of vent fan issues, start with a simple reset. Sometimes unplugging your microwave and plugging it back in can fix a microwave fan that won't turn off. However, if the microwave fan still runs constantly after the reset, you may have a bigger problem.

Cleaning or replacing the microwave's filters can also stop a microwave vent fan that turns on by itself. Depending on your microwave model, you may need to clean internal and external vents.

You should also check for excess steam or heat, which can happen if you've used the stovetop a lot that day. High humidity can make your vent fan kick on to prevent overheating.

When in doubt, call an appliance tech and explain that your microwave exhaust fan is not working the way it should. Your tech can test the temperature sensor and check the microwave's wiring.