BoilerFaultCodesYour boiler has detected that a safety sensor is stuck, preventing it from starting its ignition sequence for heating or hot water.
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 does not open
What causes this fault?
This fault occurs when the boiler's internal safety sensor incorrectly senses that the fan is already running before the ignition sequence has even started. The most common reason is that the air pressure switch has become stuck in the 'on' position or there is a blockage in the internal tubes that prevents the sensor from resetting itself to zero.
This fault can be more frequent during periods of high wind or extreme cold, as pressure imbalances or debris in the external flue can interfere with how the sensor detects airflow.
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 C4 — no call-out fees, no surprises.
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