Your boiler has stopped working because it cannot properly drain away waste water or there is a blockage in the ventilation system.

Fault: Flue non-ret. valve/condens. pump - No flue non-return flap feedback; condensate pump overflow

What causes this fault?

This fault happens when waste water from the combustion process cannot escape because the condensate pump has failed or the drainage pipes are blocked. It can also be triggered if the safety flap in the exhaust flue is stuck, which prevents the boiler from safely venting gases. When the system detects that water is backing up or air cannot flow correctly, it shuts down to prevent internal flooding or safety issues.