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Air Conditioner Temperature sensors (thermistors): What It Does and Signs It's Failing

Small sensors on wires clipped to pipes or sitting in the airflow. The indoor unit has a room air sensor and coil or pipe sensors; the outdoor unit has outdoor air, coil, discharge and compressor sensors.

Who should replace it

Technician job

Related codes

43 error codes on this site point to this part.

How it works

Each sensor's resistance changes with temperature, and the board reads it. If one reads open or shorted, the board usually stops the unit and shows a sensor code.

Signs it is failing

Sensor codes point here first, but most brands also list loose connectors and the control board. High discharge or compressor temperature codes are different: Fujitsu ties those to real overheating from low refrigerant, a restriction or poor outdoor airflow.

Can you replace it yourself?

Reset power and clean the filters; for outdoor sensor codes, clear leaves, snow and debris from around the outdoor unit too. Indoor sensors are behind the front panel and wired to the indoor board. Outdoor sensors are clipped to the pipes and wired into the outdoor unit's electrical compartment, next to the inverter electronics, so testing and replacement belong to a tech.

Always unplug the air conditioner or switch off its breaker before opening any panel, and turn off water or gas where relevant.

Buying the right part

Thermistors differ by location and resistance value. The tech will match them to your model.

Error codes linked to the temperature sensors (thermistors)

Last reviewed 2026-10-19.

Sources we checked