Your boiler has detected that the water pressure inside the system is inconsistent, which is preventing it from heating your home reliably.

DIY fix possible

Check your boiler's pressure gauge to see if it reads below 1 bar.Locate the external filling loop (usually a silver flexible hose) connected to the boiler pipes.Open the valves on the filling loop to allow cold mains water into the system until the gauge reaches 1.5 bar, then securely close the valves.