BoilerFaultCodesVaillant S.4 Fault Code
Also written as S4 — same fault.
The Vaillant S.4 fault code means: your boiler is currently operating normally and the burner has successfully lit to provide heat or hot water.. It appears on 28 Vaillant models in our database.
Technical description: Burner ignited
What causes the Vaillant S.4 fault?
This is not actually a fault, but a status message indicating that the boiler’s internal sequence is progressing correctly. It appears when the gas gas has successfully ignited and the burner is currently producing heat for your radiators or hot water. If your boiler seems stuck on this code without producing heat, it may be waiting for a signal from the thermostat or internal sensors.
DIY fix possible
No action is required as this is a status code, not an error.Monitor the boiler to ensure it transitions to other 'S' codes as demand changes.If the code persists without heating being delivered, check that your thermostat is turned up.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Vaillant models that show the S.4 fault code
Click a model for the full fault code list and model-specific repair notes.
- aquaPLUS
- Atmomax
- Combi Compact 242EB
- Combi Compact 282EB Repair: £120-350
- ecoMAX 613
- ecoMAX 618
- ecoMAX 622
- ecoMAX 635
- ecoMAX 824E
- ecoMAX 828E
- Ecomax Condensing 600/800 series
- ecoMAX System 612
- ecoTEC
- ecoTEC 65kW
- ecoTEC exclusive
- ecoTEC plus
- ecoTEC Plus 615
- ecoTEC Plus 620
- ecoTEC Plus 625
- ecoTEC Plus 630
- ecoTEC Plus 635
- ecoTEC Plus 65
- ecoTEC Plus 832
- ecoTEC Plus 836
- ecoTEC Plus 840
- ecoTEC Plus 938
- ecoTEC plus Aquaplus
- ecoTEC sustain
Source
Vaillant S.4 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