Your heat pump has stopped because a sensor that measures the temperature of the water leaving the outdoor unit is giving an incorrect reading.

Outdoor Unit, Flow Thermistor (Tw_out) Error

What causes this fault?

This fault occurs when the heat pump’s internal computer receives an impossible or missing temperature reading from the water outlet pipe. The most common reason is that the sensor has physically failed due to wear and tear, or the electrical connection between the sensor and the main control board has become loose or damaged.