S.04 Low

Vaillant S.04 Fault Code

Also written as S04 — same fault.

The Vaillant S.04 fault code means: your boiler is currently working perfectly and is simply telling you that it is busy heating your radiators.. It appears on 19 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: Status code: product is working correctly (heating mode)

What causes the Vaillant S.04 fault?

This code appears when the boiler’s internal controls have successfully triggered the burner to provide heat for your radiators. It is a normal status message indicating that the diagnostic system is monitoring a standard heating cycle. There is no physical fault; the boiler is simply confirming it is responding to your thermostat's request for warmth.

You will see this code much more frequently during the autumn and winter months when your central heating is scheduled to run regularly.

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.04 fault code

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

Source

Vaillant S.04 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 31 August 2026