Your boiler has detected that the water temperature difference between the flow and return pipes is too high, which means the water isn't circulating through your radiators properly.

flow/return ∆T error (2nd CH)

What causes this fault?

This fault occurs when the boiler's internal computer experiences a software glitch or loses communication with the sensors and valves it controls. The most common reason is a momentary power fluctuation or a breakdown in the wiring connections that prevents the main control board from processing data correctly.