Your heat pump has stopped working because a sensor that monitors the temperature of the internal heating fluid has developed a fault.

Plate Heat Exchanger Outlet Refrigerant Thermistor (T2) Error

What causes this fault?

This fault occurs when the system's internal control board loses the ability to read the temperature from a specific sensor on the heat exchanger. This usually happens because the sensor itself has failed due to wear and tear, or because the electrical wiring connecting it to the brain of the unit has become loose or damaged.