ActronAir Outdoor Defrost Sensor D4/5 103AT 1.2m PVC monitors outdoor coil temperature to trigger and terminate the defrost cycle on Classic Series residential split ducted systems, using a 103AT NTC thermistor element in 1.2m PVC-sheathed cable that clips directly to the outdoor coil fins. When this sensor fails or reads out of range, the system either runs continuous defrost -- washing heat from the outdoor coil into the ambient air -- or never defrosts, causing progressive coil ice build-up and loss of heating capacity in cold weather. Isolate power at mains before replacing any outdoor refrigeration sensors.