Your boiler has stopped working because the water isn't circulating properly between the pipes, causing a sudden and unsafe temperature gap.

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

What causes this fault?

This fault occurs when the water inside your boiler heats up too quickly because it isn't moving around the system fast enough. The most common reason is a failing pump that can no longer push the water through the pipes, or a blockage in the heat exchanger that is restricting flow. When the boiler detects such a large temperature difference between the water going out and the water coming back, it shuts down to prevent internal damage.