Your boiler has shut down because it has detected that the water leaving the unit is getting too hot, which is usually caused by a circulation problem.

When the Outlet Temperature > 90°C for 5s, the Slave board shuts down

What causes this fault?

This fault occurs when the water inside the boiler heats up too quickly and cannot be moved away into your radiators or hot water cylinder. The most common reason is a failed circulation pump or a blockage in the system, such as a build-up of sludge or limescale, which prevents the heat from being safely dispersed.