S.13 Low

Vaillant S.13 Fault Code

Also written as S13 — same fault.

The Vaillant S.13 fault code means: your boiler is currently trying to light the flame to provide hot water for your taps or shower.. It appears on 36 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: DHW mode: Ignition. The ignition for domestic hot water mode is activated.

What causes the Vaillant S.13 fault?

This isn't actually a fault, but a status message indicating that you have turned on a hot tap and the boiler is starting its ignition process. It occurs because the internal sensor has detected water flow and is now sparking the burner to heat that water for your shower or sink. If the code stays on screen without the water getting hot, it may suggest an issue with the gas supply or the ignition electrodes.

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.13 fault code

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

Source

Vaillant S.13 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