Your boiler has too much water in the system, which puts extra stress on the internal parts and may cause it to shut down for safety.

Primary System Water Pressure Too High

What causes this fault?

This fault occurs when the water pressure inside your system has risen above the safe operating limit. The most common reason is that the filling loop valve used to top up the boiler has been left slightly open or is leaking water into the system. It can also be caused by a failure in the expansion vessel, which is meant to handle the natural pressure increases that happen when water is heated.