Your heat pump has stopped working because a sensor that monitors the temperature of the water returning to the outdoor unit has failed or detected an error.

Outdoor Unit, Return Thermistor (Tw_in) Error

What causes this fault?

This fault occurs when the heat pump's control system loses a reliable signal from the sensor that measures water temperature coming back from your radiators. This is usually caused by a physical failure of the internal sensor probe or a loose connection in the wiring that carries the data. Without this information, the unit shuts down to prevent itself from overheating or running inefficiently.