S.09 Low

Vaillant S.09 Fault Code

Also written as S09 — same fault.

The Vaillant S.09 fault code means: your boiler is currently performing a routine self-check to ensure it is burning fuel efficiently, and this usually only lasts for a few moments.. It appears on 1 Vaillant model in our database.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description: Product starts by fully calibrating the combustion regulation.

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Vaillant models that show the S.09 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Vaillant S.09 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026