BoilerFaultCodesYour boiler has stopped working because it cannot safely confirm that the fan is pushing exhaust fumes out of the flue.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Differential air pressure switch open, Fan - off
What causes this fault?
This fault occurs when the boiler's internal safety sensor cannot detect the necessary airflow required to safely expel exhaust gases. It is usually caused by a failing fan motor that isn't spinning fast enough, a perished rubber tube, or a blockage within the flue pipe itself.
This issue can be more frequent during periods of high winds, as strong gusts can create back-pressure in the flue that prevents the pressure switch from closing correctly.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
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A boiler cover plan would cover faults like C1 — no call-out fees, no surprises.
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