Your boiler has detected that the exhaust gases are getting too hot, so it has shut down as a safety precaution to prevent internal damage.

Flue gas temperature is too high.

What causes this fault?

This fault occurs when the internal heat exchanger cannot transfer heat into the water efficiently, causing the waste gases to reach a dangerous temperature. The most common reason is a buildup of limescale or debris inside the heat exchanger, but it can also be caused by a faulty sensor or a damaged burner seal allowing heat to escape where it shouldn't.