Your boiler has detected that water is leaving the unit much hotter than it is returning, which usually means the water isn't circulating through your radiators fast enough.

Temperature difference between the supply sensor and return sensor too large.

What causes this fault?

This fault occurs when the heat generated by the burner isn't being carried away to your radiators quickly enough, causing heat to build up inside the unit. The most common reason is a failing pump that can no longer push the water around your home, though a blockage caused by sludge or debris in the system can also restrict this flow. If the water stays inside the boiler for too long, it overheats and the system shuts down for safety.