BoilerFaultCodesVaillant S.91 Fault Code
Also written as S91 — same fault.
The Vaillant S.91 fault code means: your boiler is currently set to a 'showroom' mode which allows the display to work for demonstration purposes but prevents the burner from actually firing to provide heating or hot water.. It appears on 23 Vaillant models in our database.
Technical description: Maintenance:Demomode
What causes the Vaillant S.91 fault?
This fault occurs when the boiler's software is accidentally placed into 'Showroom Mode', which is a setting intended for shop displays only. It means the control board thinks it should only light up the screen for demonstration rather than actually igniting the gas to heat your home. This is usually caused by a technician forgetting to deactivate the setting during installation or a mistake within the service menu.
DIY fix possible
Press and hold the 'i' and '+' buttons simultaneously for about 5 seconds.The screen should show 'd.00' or similar; use the '+' button to navigate to 'd.91'.Press the 'i' button to view the setting; if it shows '1', change it to '0' using the '-' button.Press and hold 'i' to save the change, then press 'i' and '+' together to exit.
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Vaillant models that show the S.91 fault code
Click a model for the full fault code list and model-specific repair notes.
- ecoFIT Pure 412 Repair: £80-150
- ecoFIT Pure 418 Repair: £80-120
- ecoFIT Pure 425 Repair: £80-150
- ecoFIT Pure 435 Repair: £90-150
- ecoFIT Pure 825 Repair: £80-120
- ecoFIT Pure 830 Repair: £80-120
- ecoFIT Pure 835 Repair: £80-120
- ecoFIT sustain 615
- ecoFIT sustain 618
- ecoFIT sustain 630
- ecoTEC plus 412 Repair: £80-150
- ecoTEC plus 415 Repair: £80-150
- ecoTEC plus 418 Repair: £80-150
- ecoTEC plus 424 Repair: £80-150
- ecoTEC plus 430 Repair: £80-150
- ecoTEC Plus 610 Repair: £80-150
- ecoTEC Plus 620 Repair: £0-80
- ecoTEC Plus 625 Repair: £0-80
- ecoTEC Plus 635 Repair: £0-80
- ecoTEC Plus 840 Repair: £0-80
- ecoTEC sustain Repair: £0-80
- Home Combi
- Home System Repair: £0-80
Source
Vaillant S.91 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