Your boiler has sensed that water is not circulating through the system properly, causing a large temperature gap between the pipes leaving and returning to the unit.

Temperature difference between flow/return NTC too large

What causes this fault?

This fault occurs when the water inside the boiler heats up too quickly because it isn't moving away into your radiators fast enough. The most common reasons are a failed internal pump that has stopped spinning or a blockage in the pipework that is restricting the flow of water. Either way, the boiler shuts down as a safety precaution to prevent the heat exchanger from overheating.