Your boiler has detected that the water is heating up too quickly or isn't moving through the system properly, causing it to overheat and shut down for safety.

DIFF GREATER THAN 50°C CHECK ISOLATION VALVES ARE OPEN CHECK PUMP CHECK RAD VALVES OPEN CHECK SYSTEM BLOCKAGES

What causes this fault?

This fault happens when heat cannot escape the boiler quickly enough, causing the internal temperature to spike dangerously fast. It is usually caused by a pump failure that stops water from moving or a blockage in the pipes, such as a buildup of sludge or closed valves, that prevents circulation. Effectively, the boiler is producing heat but has nowhere to send it, so it shuts down to prevent damage.