Your boiler has detected that the water pressure in your system is incorrect and it cannot safely maintain the correct level.

DIY fix possible

Locate the external filling loop, which is a silver braided hose connecting two pipes beneath the boiler.Open the small valve or taps on the filling loop to allow cold mains water to enter the system.Watch the pressure gauge on the front of the boiler and close the valves once the needle reaches 1.5 bar.