When an over-the-range microwave fan turns on by itself, it's usually the automatic vent feature: many models have a heat sensor that starts the fan when the cooktop below gets hot, to protect the microwave's electronics. On many of these models you can't switch it off until the area cools. It's a concern only if the fan runs with the cooktop cold or won't respond at all.
Quick diagnosis
| Symptom | Likely cause | First thing to try |
|---|---|---|
| Fan starts while you cook on the cooktop | Automatic heat-sensing vent | Normal; let it run |
| Fan won't turn off with the button | Heat sensor still hot | Wait for the area to cool |
| Fan runs briefly after microwave cooking | Cooling run on some models | Normal; it stops on its own |
| Fan runs all day with the cooktop cold | Stuck sensor, relay or control | Switch off the breaker for a minute, then restore power |
| Fan doesn't run at all | Failed motor, switch or control | Call for service |
| Rattling or grinding from the vent | Loose grease filter or a worn motor | Check that both grease filters are seated |
A closer look
Automatic heat sensing
An over-the-range microwave sits right above the heat and steam of the cooktop. To keep its control board from overheating, many models have a thermal sensor near the bottom that turns the vent fan on when it gets hot. Searing, boiling big pots or baking in an oven that vents up at the back of the cooktop can all trigger it.
Cooling after use
Some microwaves also run a fan for a short time after cooking to cool internal parts. It stops on its own.
When it won't stop
If the fan runs for hours with the cooktop cold, a sensor, relay or the control board may be stuck. Power cycling can clear a glitch. Over-the-range microwaves usually plug into an outlet in the cabinet above, but the breaker is often easier. Don't open the microwave cabinet: microwaves contain a high-voltage capacitor that can hold a dangerous charge even after unplugging.
Grease filters and airflow
The metal grease filters under the microwave catch grease before it reaches the fan. Clogged filters make the fan work harder and louder, and a loose filter can rattle. Many are washable in warm, soapy water; check your manual.
Who runs into this most
Cooks who sear, stir-fry or boil often on the front burners, households that run several burners at once, and kitchens where the microwave sits low over the range.
Prevention checklist
- •Clean the grease filters about once a month with regular cooking
- •Use the vent fan during cooking instead of waiting for it to start itself
- •Keep filters seated and latched
- •Let the fan run until it shuts off
Time to call a pro
- •The fan runs constantly with the cooktop cold
- •The fan won't run on any speed
- •Grinding, squealing or burning smells from the vent
- •The microwave shuts off or shows errors while the cooktop is in use
Travis's rule of thumb: If the stove's hot, the fan's doing its job. If the stove's cold and the fan won't quit, that's a service call.
Fan running all day over a cold stove? Start with our GE appliance repair page or call ProFix Houston at (832) 552-1615. If the fan button itself does nothing, see why a microwave keypad stops working.