BoilerFaultCodesYour boiler is struggling to start because it cannot confirm that the fan is pushing enough air through the flue to safely clear fumes.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Air pressure switch check / Fan run check
What causes this fault?
This fault occurs when the boiler's safety sensors cannot detect sufficient airflow to safely exhaust burnt gases to the outside. The most common reasons are a failing fan that isn't spinning fast enough, a blockage in the flue pipe, or thin rubber tubes inside the unit becoming perished or blocked with moisture.
This fault is more common during periods of high winds, as strong gusts can create back-pressure in the flue that prevents the air pressure switch from closing correctly.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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A boiler cover plan would cover faults like S.33 — no call-out fees, no surprises.
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