Your boiler has too much water in the system, which causes it to shut down for safety.

High water pressure error

What causes this fault?

This fault occurs when the water pressure inside your boiler exceeds a safe limit, usually because the heating system has been overfilled via the manual filling loop. It can also be caused by a faulty tap on that filling loop leaking water into the system or an internal failure of the expansion vessel which normally helps manage pressure changes.