Your heat pump's external temperature sensor is giving a faulty reading, which means the system cannot accurately adjust the heating based on the weather outside.

Outdoor Unit, Air to Refrigerant Heat Exchanger Outlet Thermistor (T3) Error

What causes this fault?

This fault occurs when the heat pump loses communication with the sensor that monitors the temperature of the air outside. The most common reason is that the sensor itself has developed an electrical fault or the wiring has become corroded or loose due to exposure to the elements. Without this data, the system cannot calculate how hard it needs to work to keep your home warm.