Your boiler has shut down because a sensor has detected that the exhaust gases are getting too hot, which is a safety measure to prevent damage.

Flue NTC Operated

What causes this fault?

This fault occurs when a safety sensor detects that the gases leaving your boiler's chimney are reaching dangerously high temperatures. The most common cause is a buildup of debris or limescale inside the heat exchanger, which prevents heat from being transferred to your water and instead sends it out through the flue. It can also be triggered by a faulty sensor sending an incorrect reading to the boiler's computer.