High
S.42 on Vaillant ecoTEC Pro 30

Your boiler has detected a blockage or communication error in its venting system, which is preventing it from safely starting up.

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.

What the manual says

Manufacturer manual

Flue non-return flap return signal blocks burner operation (only in connection with VR 40 accessories) or condensate pump faulty, heat requirement is blocked

From this model's manual
Verified against the Vaillant Ecotec Pro 30 installation & service manual, page 35.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

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

S.41View →F.22View →

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