Your boiler has shut down because the internal exhaust gases have reached a temperature that is dangerously high, posing a risk of damage to the unit.

What causes this fault?

This fault occurs when the internal sensors detect that the exhaust gases leaving the boiler are far too hot, which can lead to melting or warping of internal components. The most common reason is a buildup of limescale or sludge inside the heat exchanger, which prevents heat from being transferred to your water properly and forces it out through the flue instead.