BoilerFaultCodesVaillant S.04 Fault Code
Also written as S04 — same fault.
The Vaillant S.04 fault code means: your boiler is operating normally and is currently in the process of heating up after successfully lighting the burner.. It appears on 45 Vaillant models in our database.
Technical description: Burner ignited (heating operation)
What causes the Vaillant S.04 fault?
This code appears when the boiler's internal controls successfully detect a flame and confirm that the burner has lit correctly. It is a standard status message indicating that the system is now actively burning gas to heat your water or radiators. You do not need to take any action because this shows the boiler is working exactly as it should.
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.
- Atmomax
- ecoFIT Pure 412
- ecoFIT Pure 418
- ecoFIT Pure 425
- ecoFIT Pure 435
- ecoFIT Pure 825
- ecoFIT Pure 830
- ecoFIT Pure 835
- ecoFIT sustain 615
- ecoFIT sustain 618
- ecoFIT sustain 630
- ecoFIT sustain 830
- ecoTEC exclusive 627
- ecoTEC plus 412
- ecoTEC plus 415
- ecoTEC plus 418
- ecoTEC plus 424
- ecoTEC plus 430
- ecoTEC Plus 610
- ecoTEC Plus 620
- ecoTEC Plus 625
- ecoTEC Plus 630
- ecoTEC Plus 635
- ecoTEC plus 637
- ecoTEC Plus 826
- ecoTEC Plus 832
- ecoTEC Plus 840
- ecoTEC Plus 938
- ecoTEC plus aquaPLUS
- ecoTEC Pro
- ecoTEC Pro / ecoTEC Plus
- ecoTEC Pro 24
- ecoTEC Pro 28
- ecoTEC Pro 30
- ecoTEC sustain
- Home Combi
- Home System
- Thermocompact
- Turbomax 824/828
- Turbomax Plus
- Turbomax Plus 824/828E
- Turbomax Plus 828
- Turbomax Plus 828 E
- Turbomax Plus VUW 242E
- Turbomax Plus VUW 242EH
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.
- Vaillant ecoFIT Pure 425 manual (PDF)
- Vaillant ecoFIT Pure 835 manual (PDF)
- Vaillant ecoFIT sustain 630 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026