Your boiler has shut down because the exhaust fumes have become dangerously hot, which could indicate a blockage or a heat exchanger problem.

What causes this fault?

This fault occurs when the internal sensors detect that the exhaust gases are reaching dangerously high temperatures, triggering an automatic shutdown to prevent heat damage. The most common cause is a build-up of debris or limescale inside the main heat exchanger, which prevents heat from being transferred to your water and pushes it out through the flue instead. It can also be caused by a faulty sensor giving an incorrect reading or a physical blockage in the chimney-like flue pipe.