Your boiler has detected an issue with the drainage system that removes leftover liquid from the exhaust pipe or a fault with the safety flap inside the flue.

Flue non return flap condensate pump error

What causes this fault?

This fault occurs when the boiler's internal safety sensors detect that water is backing up in the flue system or that the mechanical flap designed to stop exhaust fumes from returning is stuck. The most common reason is a failed condensate pump that can no longer push waste liquid away, or debris interfering with the flue flap's movement. Because this affects how exhaust gases and liquids leave the unit, the boiler shuts down as a safety precaution.