Your boiler has shut itself down because the air being pushed out of the chimney pipe has become dangerously hot.

Safety switch-off: Flue gas temperature too high

What causes this fault?

This fault occurs when the internal temperature of the exhaust gases exceeds safe limits, triggering a safety sensor to prevent damage or fire. The most common reason is a buildup of debris or limescale inside the heat exchanger, which prevents heat from being transferred to your water and instead pushes it out through the flue. It can also be caused by a faulty sensor sending incorrect readings or a lack of proper water flow through the system.