High
S.21 on Vaillant Atmomax

Your boiler is trying to provide hot water, but the internal fan is failing to start up properly to clear the exhaust gases.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

DHW mode – fan start up error

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.

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Related fault codes

F.23View →F.32View →S.32View →

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