Your boiler has detected that a safety flap in the flue pipe is stuck closed, preventing exhaust gases from escaping.

Response from accessory module or defective condensate pump is blocking burner operation

What causes this fault?

This fault occurs when a mechanical flap inside the exhaust system fails to open, which safely prevents outdoor air or fumes from flowing back into the boiler. The most common cause is the flap becoming physically stuck due to a build-up of debris or the fan failing to generate enough pressure to push it open.