Your boiler has switched itself off because the exhaust gases have become too hot, which usually suggests a blockage or a fault in the flue system.

What causes this fault?

This fault occurs when the internal safety sensor detects that the exhaust gases leaving the boiler are reaching dangerously high temperatures. The most common cause is a significant buildup of debris or limescale inside the heat exchanger, which prevents heat from transferring to your water and instead sends it out through the flue. It can also be triggered if the flue pipe itself is blocked or if the internal seals have perished.