Your boiler has shut down because the exhaust gases have reached a temperature that is too high, indicating a potential blockage or a serious internal circulation issue.

Flue NTC Operated / overheat of the flue system

What causes this fault?

This fault occurs when the safety sensor detects that the exhaust gases leaving your boiler are dangerously hot, which triggers an immediate shutdown to protect the internal plastic components. The most common reasons for this are a severe blockage in the heat exchanger preventing heat from transferring to the water, or a failure in the internal pump causing the water to sit still and overheat.