Your boiler has detected that there is either insufficient water circulating inside it or that its internal temperature sensors are failing to register a rise in heat when the burner starts.

Dry fire - Flow/return temperature sensor change too small

What causes this fault?

This fault occurs when the boiler's internal computer registers that the burners are on, but the temperature sensors fail to detect a significant rise in heat within the water. The most common reason is a lack of water circulating through the system, often caused by a pump failure, a blockage, or even just too much air trapped in the pipework. Because the boiler cannot move the heat away from the burner quickly enough, it shuts down to prevent 'dry firing' and damaging the internal components.