BoilerFaultCodesVaillant 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 43 Vaillant models in our database.
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.
- Atmomax Repair: £100-£250
- 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
- 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 826
- ecoTEC Plus 832
- ecoTEC Plus 840
- ecoTEC Plus 938
- ecoTEC plus aquaPLUS
- ecoTEC Pro
- ecoTEC Pro / ecoTEC Plus Repair: £120-350
- ecoTEC Pro 24
- ecoTEC Pro 28
- ecoTEC Pro 30
- ecoTEC sustain
- Home Combi
- Home System
- Thermocompact
- Turbomax 824/828
- Turbomax Plus Repair: £120-250
- Turbomax Plus 824/828E
- Turbomax Plus 828
- Turbomax Plus 828 E
- Turbomax Plus VUW 242E
- Turbomax Plus VUW 242EH
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.
- 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