ActronAir Coil Sensor (Indoor) 1.2m 2-Pin JST Connector measures indoor coil temperature on ESP Plus, AIRES, and Advance R32 split ducted systems, providing the coil temperature data the indoor PCB uses to manage coil operating limits, frost protection, and demand defrost control. A faulty indoor coil sensor causes erratic frost protection responses -- either allowing coil icing that restricts airflow, or triggering unnecessary defrost cycles that interrupt heating mode -- and typically generates a specific sensor fault code on the wall controller. Isolate power at mains before replacing coil temperature sensors.