Your boiler has detected that the water pressure inside your heating system has dropped too low to operate safely.

DIY fix possible

Locate the filling loop (a flexible silver hose) beneath your boiler.Open the valves at both ends of the filling loop to allow water to enter the system.Watch the pressure gauge on the front of the boiler until it reaches the green zone (typically between 1.0 and 1.5 bar).Close both valves tightly and remove the filling loop if it is detachable.