S.21 Low

Vaillant S.21 Fault Code

Also written as S21 — same fault.

The Vaillant S.21 fault code means: your boiler is performing a routine initial check of its internal air fan before it attempts to ignite the burner.. It appears on 41 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: Fan running

What causes the Vaillant S.21 fault?

This is a normal status message rather than a breakdown, occurring every time the boiler prepares to heat your water or radiators. It happens when the control board sends power to the internal fan to clear out any residual gases and ensure a safe airflow before the burner lights. If the boiler stays on this code without firing, it usually means the fan is unable to reach the required speed or the air pressure sensor isn't detecting the flow.

DIY fix possible

No action is required as this is a normal status message, not a fault code.Wait a few minutes to see if the boiler moves on to the next stage of its operation.If the code stays on screen for a long time without the boiler firing, try resetting the boiler once.

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Vaillant models that show the S.21 fault code

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

Source

Vaillant S.21 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