Your boiler has switched itself off because the waste gases are getting too hot, which is usually caused by a lack of water circulating through the system.

Flue gas temperature too high

What causes this fault?

This fault occurs when the internal sensors detect that the exhaust gases are getting dangerously hot, triggering a safety shutdown to prevent damage. The most common reason is that heat isn't being carried away from the burner quickly enough, usually because the water isn't circulating properly or there is a blockage within the boiler's pipework.