BoilerFaultCodesVaillant S.23 Fault Code
Also written as S23 — same fault.
The Vaillant S.23 fault code means: your boiler is having trouble lighting the flame when you turn on your hot water taps.. It appears on 46 Vaillant models in our database.
Technical description: DHW ignition error
What causes the Vaillant S.23 fault?
This fault occurs when the boiler fails to establish or sustain a flame specifically while trying to heat your tap water. It is usually caused by a component failure that prevents the spark from jumping or the gas from flowing correctly, meaning the system cannot safely ignite to provide heat. The most common reasons include a worn-out ignition probe or a faulty gas valve that isn't opening as it should.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Vaillant models that show the S.23 fault code
Click a model for the full fault code list and model-specific repair notes.
- Atmomax Repair: £120-£350
- ecoFIT Pure 412
- ecoFIT Pure 418
- ecoFIT Pure 425
- ecoFIT Pure 435
- ecoFIT Pure 825
- ecoFIT Pure 830
- ecoFIT Pure 835
- ecoMAX Repair: £120-250
- 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 Repair: £120-350
- ecoTEC Plus 610
- ecoTEC Plus 615
- ecoTEC Plus 620
- ecoTEC Plus 625
- ecoTEC Plus 630
- ecoTEC Plus 635
- ecoTEC Plus 826
- 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 Repair: £120-250
- 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.23 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 412 manual (PDF)
- Vaillant ecoFIT Pure 835 manual (PDF)
- Vaillant ecoMAX manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026