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

DIY fix possible

Locate a radiator bleed valve and use a key to slowly release a small amount of water into a cloth or bowl.Monitor the pressure gauge on the boiler until it drops back into the green zone (typically 1.0 to 1.5 bar).Ensure the black filling loop handles under the boiler are tightly closed to prevent more water from entering.