BoilerFaultCodesWhat the manual says
Manufacturer manual(837/637 only) | 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 is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Avoid the next call-out bill
A cover plan would handle faults like F.33: no call-out fees, no surprise bills.
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