Your boiler's internal water pressure is currently too high, which means the system has taken in too much water and needs to be brought back down to a safe level to operate correctly.

DIY fix possible

Turn off the boiler and allow it to cool down completely.Locate a radiator in your home and attach a bleed key to the valve.Open the valve slightly to let a small amount of water out into a container until the pressure gauge on the boiler drops to the recommended level (usually between 1 and 1.5 bar).Close the valve and check the boiler display to see if the error has cleared.