A high limit switch is a critical safety sensor located inside a furnace that monitors the temperature of the Heat Exchanger. If the air inside becomes dangerously hot—usually due to a failed blower motor or a dirty filter—the switch automatically cuts power to the gas valve to prevent a fire. In Mobile Home Gas Furnaces, this switch is the primary defense against structural damage to the home. If your furnace is "short-cycling" (turning on and off rapidly), it is often because the high limit switch is being triggered. You can find replacement switches and diagnostic tools in our Parts collection. Our ACIq heating units utilize high-precision switches that are calibrated for the specific thermal limits of the cabinet. Regularly checking your Air Handlers airflow is the best way to prevent this switch from ever needing to act. It is a simple but life-saving component in every ducted heating system.