BoilerFaultCodesVaillant S.46 Fault Code
Also written as S46 — same fault.
The Vaillant S.46 fault code means: your boiler has detected very cold temperatures and has automatically turned itself on at a low setting to prevent the internal pipes from freezing.. It appears on 33 Vaillant models in our database.
Technical description: Frostprotectionmode(Comfort):Minimumload
What causes the Vaillant S.46 fault?
This occurs when the boiler's internal sensors detect that the water temperature has dropped to a level where there is a risk of freezing. To protect itself, the boiler automatically fires up at a minimum heat level to circulate warm water through the internal components. This is a normal safety function designed to prevent the heat exchanger from cracking due to ice expansion.
This status is almost exclusively seen during the winter months or during sudden overnight cold snaps when temperatures drop near freezing.
DIY fix possible
Check that your external condensate pipe is not frozenEnsure radiators are turned on and TRVs are open to allow water circulationWait for the boiler to finish the frost protection cycle; it will return to normal once the temperature rises
Find a Gas Safe engineer
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A cover plan would handle faults like S.46: no call-out fees, no surprise bills.
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Vaillant models that show the S.46 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
- 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 Repair: £120-250
- ecoTEC Plus 832
- ecoTEC Plus 840
- ecoTEC Plus 938
- ecoTEC Pro Repair: £80-220
- ecoTEC Pro / ecoTEC Plus Repair: £100-£250
- ecoTEC Pro 24 Repair: £80-220
- ecoTEC Pro 28 Repair: £80-220
- ecoTEC Pro 30 Repair: £120-250
- Home Combi
- Home System Repair: £0-120
Source
Vaillant S.46 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