Your boiler has detected trapped air inside the system, which is preventing water from circulating properly to heat your home.

DIY fix possible

Turn off and isolate the boiler from the power supply.Locate the bleed valves on your radiators, starting from the downstairs units and moving upstairs.Open the valves slightly with a radiator key to release trapped air until water flows steadily, then close them tightly.Check your boiler's pressure gauge and top up the system to 1.5 bar if necessary using the filling loop.Restart the boiler.