BoilerFaultCodesYour boiler has shut down because it cannot breathe properly, likely due to a blockage in the pipes leading to the outside of your home.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Flue and / or air supply duct is blocked
What causes this fault?
This fault occurs when the boiler's internal sensors detect that it cannot safely pull in fresh air or expel exhaust gases. The most common reasons include an external blockage in the flue pipe, such as a bird's nest or debris, or a mechanical failure of the fan that prevents air from moving through the system.
This fault can occur more frequently during autumn or high winds, as fallen leaves or wind-blown debris are more likely to obstruct the external flue terminal.
DIY fix possible
Check the external flue terminal on your outside wall for obstructions like bird nests, leaves, or debris.During freezing weather, check if the flue outlet is blocked by ice or snow and gently clear it.Reset the boiler by pressing the reset button once any external obstructions are removed.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like F018 — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.
Could it be time for a new boiler?
If repair costs are adding up, a new energy-efficient boiler could be the smarter long-term choice.
Affiliate links — we may earn a commission at no extra cost to you.