Wind or air is blowing back down your boiler's chimney, which is preventing the exhaust fumes from escaping safely.

Downdraught

What causes this fault?

This fault occurs when strong gusts of wind enter the flue pipe and push air back into the boiler, disrupting the internal pressure balance. The most common reason is that the flue terminal is positioned in an area exposed to high winds or is missing a protective guard, causing the safety sensors to trigger a shutdown to prevent fumes from entering the home.