Your boiler has detected that a temperature sensor has lost physical contact with the internal pipework, meaning it can no longer monitor heat levels correctly.

Central heating NTC not connected to pipe

What causes this fault?

This fault occurs when the temperature sensor responsible for monitoring your heating water becomes physically detached from the pipe it is meant to sit on. The most common reason is a snapped or weakened metal retaining clip, though it can also be caused by excessive vibration or a loose wiring plug that has vibrated free over time.