S.23 Low

Vaillant S.23 Fault Code

Also written as S23 — same fault.

The Vaillant S.23 fault code means: your boiler is currently preparing to fire up the burners to heat your water or radiators after a brief pause.. It appears on 31 Vaillant models 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: Ignition sequence (warm start)

What causes the Vaillant S.23 fault?

This is a normal part of your boiler's operating cycle rather than a mechanical failure. It occurs when the boiler has briefly paused because it reached the desired temperature and is now re-engaging the ignition process to maintain the heat for your home. The 'S' prefix indicates a status message, meaning the system is simply telling you what it is doing at that moment.

DIY fix possible

This fault may be fixable without an engineer. Check your manual first.

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Vaillant models that show the S.23 fault code

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

Source

Vaillant S.23 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 31 August 2026