Your boiler has detected that the exhaust gases are becoming dangerously hot, so it has safely shut itself down to prevent damage.

High flue temperature fault. Check for air in t

What causes this fault?

This fault occurs when the internal heat exchanger isn't transferring heat efficiently to the water, causing the leftover heat to escape into the exhaust flue at dangerous levels. The most common reasons are a buildup of debris and sludge inside the main heat exchanger or a failing sensor that is incorrectly reading the exhaust temperature.