Cooking appliance installed under the microwave oven may, under some heavy usage conditions, cause temperatures high enough to overheat some internal parts of the microwave oven.
To prevent overheating from taking place, the exhaust fan is designed to automatically turn on at a low speed if excessive temperatures occur. Should this happen, the fan cannot be manually turned off, but it will automatically turn off when the internal parts have cooled. The fan may stay on for up to an hour after the range and microwave oven controls have been turned off.