Your boiler has detected that a temperature sensor is stuck or faulty, meaning it cannot accurately monitor the water temperature to operate safely.

Flow NTC reports constant value (stuck at) | Flow NTC is defective

What causes this fault?

This fault occurs when the boiler's internal computer notices that the flow temperature sensor is sending the exact same reading regardless of whether the burner is on or off. The most common reason is that the sensor has suffered an internal electrical failure or has become covered in limescale, preventing it from detecting actual changes in water temperature.