Your boiler has detected that there is too much water in the system, causing the internal pressure to rise above safe operating levels.

Water pressure too high. The system pressure is too high.

What causes this fault?

This fault occurs when the water pressure inside your heating system exceeds the safe limit, often because the filling loop used to top up the system has been left slightly open or is leaking. It can also happen if the expansion vessel, which manages heat-related pressure changes, has failed or if the system has been significantly overfilled during recent maintenance.