Your boiler has detected that there is too much water pressure in the system, which can cause the safety valve to leak or damage internal components.

DIY fix possible

Check that the filling loop valves are fully closed and not letting in waterBleed water from a radiator using a bleed key to reduce the system pressureMonitor the pressure gauge to ensure it returns to between 1.0 and 1.5 barReset the boiler once the pressure is within the normal operating range