BoilerFaultCodesYour boiler has stopped working because it cannot confirm that harmful exhaust fumes are being safely cleared away through the flue pipe.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Main heat exchanger anti freeze protection mode due to no switching signal from the air pressure switch | Check air pressure switch, fan, fan supply or flue system
What causes this fault?
This fault occurs when the boiler's internal safety sensor doesn't detect the necessary airflow required to safely expel exhaust gases. The most common reasons are a failing fan that cannot spin fast enough, a blocked or damaged flue pipe, or a faulty sensor that is incorrectly reporting a lack of pressure.
This fault is more frequent during periods of high winds or extreme cold, as wind gusts can disrupt flue pressure and ice buildup can physically block the air intake.
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 F.33 — no call-out fees, no surprises.
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