Your boiler has shut down because a safety sensor has detected that the burner area is getting too hot or is failing to register a secure connection.

Burner thermostat is open or not connected; check the thermostat for the switching point 107˚C. Check the burner cover for defects, it is possible that the flame will extend to the rear-side of the burner cover. Check the sealing between the mixing chamber and the heat exchanger and check the connection of the burner housing to the heat exchanger.

What causes this fault?

This fault occurs when a safety sensor detects that the area around the burner is overheating, usually because heat is escaping where it shouldn't. The most common reason is a perished seal or a gap between internal components, which allows hot combustion gases to leak out and trigger the safety shut-off. It can also be caused by the sensor itself failing or a loose wiring connection preventing the boiler from confirming it is safe to fire.