S.03 Low

Vaillant S.03 Fault Code

Also written as S03 — same fault.

The Vaillant S.03 fault code means: your boiler is currently going through its routine process of lighting the flame to start heating your water.. 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 (heating operation)

What causes the Vaillant S.03 fault?

This status code appears when the boiler is actively trying to light the gas burner to provide heating or hot water. It represents a normal part of the startup cycle where the internal computer checks the safety sensors before opening the gas valve and creating a spark. If this code stays on screen for a long time without the boiler firing up, it usually suggests a delay in the ignition process.

DIY fix possible

No action required; this is a normal status message and not a fault.Wait for the boiler to complete the ignition sequence.Ensure your thermostat is calling for heat or hot water.

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Vaillant models that show the S.03 fault code

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

Source

Vaillant S.03 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