Your boiler has sensed that the exhaust gases are getting too hot and has shut down to prevent internal damage.

Flue gas temperature is too high.

What causes this fault?

This fault occurs when the internal sensors detect that the waste gases leaving your boiler have reached an unsafely high temperature. The most common reasons are a significant build-up of limescale or debris inside the heat exchanger, or a lack of water flow preventing the heat from being transferred away from the unit effectively.