Your boiler is overheating because it cannot circulate heat away from the burner fast enough, so it has safely shut itself down to prevent damage.

Excessive temperature rise

What causes this fault?

This fault occurs when the water inside the boiler heats up at an unnaturally fast rate, causing the sensors to trigger a safety shutdown before the unit overheats. The most common reason is a lack of water flow, often caused by a failing internal pump or a blockage in the system that prevents heat from being moved away from the burner quickly enough.