BoilerFaultCodesYour boiler has stopped working because it cannot confirm that the fan is safely 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 fan pressure switch
What causes this fault?
This fault occurs when the boiler’s safety sensors cannot detect any airflow, meaning it is unsure if dangerous exhaust fumes are being cleared. The most common reasons are a buildup of dirt in the fan, a blockage in the thin plastic tubes inside the casing, or the air pressure switch failing to click into place. Because the boiler cannot guarantee a safe exit for burnt gases, it shuts down immediately as a protective measure.
This issue is more frequent in autumn or after high winds, as debris such as fallen leaves or nesting material can become lodged in the external flue pipe.
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 66C — no call-out fees, no surprises.
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