Your boiler has detected that water isn't moving through the system fast enough, causing a large temperature difference that has triggered a safety shutdown.

Water shortage, temperature spread between flow and return probe too large: Pump blocked or defective, pump output too low, flow and return sensor swapped over

What causes this fault?

This fault occurs when the water inside your boiler heats up much faster than it can be moved away into your radiators. The most common reason is a failing internal pump or a buildup of debris like sludge that is physically blocking the flow of water. When the boiler detects that the outgoing water is significantly hotter than the returning water, it shuts down to prevent the unit from overheating.