Your boiler has sensed that the exhaust gases are getting too hot and has shut down as a safety precaution to prevent damage.

Flue NTC Operated (Overheat of the flue system)

What causes this fault?

This fault occurs when a temperature sensor inside the boiler detects that the exhaust gases are becoming dangerously hot, triggering an automatic shutdown to protect the internal components. The most common reasons for this are a build-up of debris or limescale inside the heat exchanger, or damaged insulation panels that are no longer keeping the heat contained where it should be.