S.91 High

Vaillant 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 14 Vaillant models in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

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.

Typical repair cost £0-80 (including labour)

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.

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.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026