What is a thermostat?

A cycling thermostat switches the heat on and off to hold a target temperature. A high-limit thermostat is a safety device that opens if things get too hot and usually resets when they cool, which produces the classic "works for a while, then stops, then works again after an hour" complaint. In refrigeration, a bimetal defrost thermostat decides when it is cold enough to warrant running the defrost heater. All of them test with a multimeter for continuity.