Your boiler has detected that a temperature sensor has stuck on a single reading, meaning it cannot accurately tell how hot the water is to safely heat your home.

Flow NTC error – flow temperature sensor signal constant value

What causes this fault?

This fault occurs when the boiler's internal computer notices that the temperature reading from the flow sensor isn't changing, even though the burner is firing. The most common reason is that the sensor has physically failed or 'seized' internally, but it can also be caused by a loose connection or damaged wiring that prevents the sensor from sending updated data.