Your boiler has shut down because it detected that the exhaust gases have become too hot, which is a safety feature to prevent damage or dangerous operation.

What causes this fault?

This fault occurs when the internal safety sensors detect that the exhaust gases leaving the unit are exceeding safe temperature limits, triggering an immediate shutdown to protect the internal components. The most common reasons are a build-up of limescale or debris inside the heat exchanger that prevents heat from being transferred to the water, or a fault with the temperature sensor itself. It can also be caused by the burner not firing correctly, leading to excessive heat buildup within the casing.