Your boiler has detected that the water pressure is too high, often caused by the filling loop being left open or a fault in the expansion system.

DIY fix possible

Check that the silver braided filling loop valves are tightly closedBleed water from a radiator using a radiator key to reduce the 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 range