Your boiler has detected that there is not enough water circulating through the system, which could be due to a leak or a mechanical issue with the pump.

Water is leaking or the pump needs replacing or freeing due to the pump running dry.

What causes this fault?

This fault occurs when the boiler detects a serious lack of water pressure or circulation, often caused by a leak within the system or a failing internal pump. If the pump is spinning without enough water (running dry) or has become seized with debris, the boiler will shut down to prevent the components from overheating or burning out. In some cases, a faulty air vent can also cause an air lock that tricks the sensors into thinking the system is empty.