Over-the-range microwave vent fan turns on by itself or won't turn off
Most OTR microwaves have a heat-protection thermostat that starts the vent fan when the cooktop below heats the unit, and on that cycle the fan cannot be switched off until the area cools. Some models also run the fan during and after microwave cooking or link the hood to the cooktop. If the fan runs in a cool kitchen, right at power-up, or for hours, suspect a thermostat stuck closed, a welded fan relay on the control board, a stuck vent key or a shorted wire.
Single source ~45 min moderateBrands: All
Before you start
- DANGER: the HV capacitor can hold a lethal charge after the microwave is unplugged
- Homeowners must not open the cabinet; internal steps are for trained techs only
- Trained techs: unplug or breaker off, discharge the HV capacitor with an insulated tool and verify 0 V before touching internal parts
- While plugged in, line voltage is present at the vent motor and board even with the fan off
- Never run the oven with the cabinet off or with interlocks bypassed
- Burning smell, arcing or a tripping breaker: unplug and stop using the unit
Diagnostic steps
Unlock all 8 steps with expected values
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