Your boiler has detected that the water pressure inside the system has risen too high, which could cause a safety relief valve to leak.

DIY fix possible

Check that the filling loop valves are fully closed and not leaking water into the systemIdentify a radiator in your home with a bleed valveOpen the bleed valve slightly to release a small amount of water until the pressure gauge on the boiler drops to between 1.0 and 1.5 bar