Your boiler has switched itself off because the water temperature inside it is rising much faster than the water temperature being sent to your radiators, usually due to a circulation blockage.

Safety switch off temperature difference too great

What causes this fault?

This fault occurs when the internal water temperature rises too quickly because the heated water cannot leave the boiler fast enough to reach your radiators. The most common reason is a pump failure or a significant blockage in the system, such as a build-up of sludge or debris, which prevents the water from circulating properly.