S.55 Low

Vaillant S.55 Fault Code

Also written as S55 — same fault.

The Vaillant S.55 fault code means: your boiler is undergoing a mandatory safety check of its internal air quality sensors and is briefly pausing to ensure it is operating safely.. 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: Waitingtime:COsensor | Waitingperiod:COsensor

DIY fix possible

Wait for the countdown to finish as this is a normal self-testEnsure there are no obstructions to the flue terminal outsideIf the code persists for more than 15 minutes, try a manual reset

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Vaillant models that show the S.55 fault code

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

Source

Vaillant S.55 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