The microwave has a base thermostat which is a safety device.
Exhaust fan on microwave won t shut off.
It cannot be turned off manually.
Microwave has automatic fan ignition setting pre programmed to keep the unit components from over heating.
When the microwave gets hot the thermostat closes.
Thus turning the fan on will keep the electronic control board from getting too hot.
If you are cooking on the cooktop it can sense the heat and the fan won t turn off until the thermostat cools.
The exhaust fan will turn on automatically during heavy use of the surface burners on the cook top if the fan thermostat senses too mush heat.
The exhaust fan thermostat is usually the failed component.
On several models of microwaves the thermostat is super sensitive causing the exhaust to run longer than needed.
Fridgidaire microwave will not turn off check the fan.
When it senses heat it turns on the exhaust fan.
It will come to help keep the controls cool inside the microwave control panel.