Sensors and thermal fuses in refrigerators/freezers are safety and control components that help keep temperatures stable and protect the appliance from overheating.

Temperature sensors (often NTC thermistors) measure the air or evaporator temperature and send data to the control board. The board uses these readings to run the compressor, fans, and defrost heater, and to detect faults. If a sensor is defective, symptoms can include unstable temperature, incorrect display readings, continuous running or frequent cycling, poor cooling, or error codes.

Thermal fuses and defrost thermostats are protective devices, usually used in the defrost/heater circuit. A thermal fuse opens permanently if temperatures rise beyond a safe limit, cutting power to the heater to prevent overheating. A defrost thermostat (bi-metal) opens/closes depending on evaporator temperature to control or limit defrost heating. Failures can cause “no defrost” (heavy ice buildup, fan hitting ice, weak airflow) or, less commonly, defrost not stopping correctly (overheating or excessive water).

Any testing or replacement involves mains voltage and internal disassembly, so the appliance should be unplugged first; if you’re not equipped to measure resistance/continuity safely, it’s best handled by a technician.