Your boiler has detected that there is too much water in the system, which puts unnecessary stress on the internal seals and components.

DIY fix possible

Check that the filling loop valves are fully closed and not leakingLocate a radiator and use a bleed key to remove water through the air ventMonitor the pressure gauge until it returns to between 1.0 and 1.5 barIf pressure continues to rise on its own, call a Gas Safe engineer