S.01 Low

Vaillant S.01 Fault Code

Also written as S01 — same fault.

The Vaillant S.01 fault code means: your boiler is currently preparing to provide heating and is going through its normal start-up cycle.. It appears on 28 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: Heating mode: Fan start-up. The fan start-up for heating mode is activated.

What causes the Vaillant S.01 fault?

This status code appears when the boiler's internal computer tells the fan to begin spinning to clear the combustion chamber and prepare for ignition. It is a normal part of the heating cycle and indicates that the system is successfully moving through its safety checks before firing the burner. If the code stays on screen without the boiler firing, it may suggest a mechanical struggle with the fan or a communication delay between internal components.

You may notice this code more frequently during the autumn when you first switch the heating back on after a long summer break.

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.01 fault code

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

Source

Vaillant S.01 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 7 July 2026