A built-in temperature sensor in some models automatically turns on the fan if the temperature below the hood gets too high. This feature is available mainly on over-the-range microwaves. The real purpose of the thermostat is to protect the microwave electronics from being damaged by high temperatures. If the temperature under the microwave is too high, the exhaust fan comes on to draw away the hot air and pull in cooler air from the rest of the kitchen. But we do not recommend this feature on range hoods because if you are cooking with oil and your pan catches fire, the exhaust fan will come on, drawing more air to the fire, fanning it, and perhaps making matters worse.