Your boiler has too much water inside it, causing the internal pressure to rise to an unsafe level.

Upper water pressure (>2.7b)

What causes this fault?

This fault occurs when the water pressure inside your heating system exceeds a safe operating limit, often because the filling loop valve has been left slightly open or is leaking water into the system. It can also happen if the expansion vessel, which manages pressure changes as water heats up, has failed or lost its air charge. In some cases, a faulty plate heat exchanger may be allowing high-pressure mains water to leak internally into the heating circuit.