BoilerFaultCodesVaillant S.46 Fault Code
Also written as S46 — same fault.
The Vaillant S.46 fault code means: your boiler has noticed an unstable flame when running at its lowest setting and is operating at a restricted level to prevent a complete breakdown.. It appears on 11 Vaillant models in our database.
Technical description: Comfort protection: Minimum load, loss of flame. Comfort protection mode for flame loss at minimum load is activated.
What causes the Vaillant S.46 fault?
This fault occurs when the boiler struggles to maintain a steady flame while running at its lowest power level. The most common cause is a dirty or worn-out sensing probe that can no longer accurately detect the fire, or an incorrect mix of gas and air preventing a consistent burn.
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.46 fault code
Click a model for the full fault code list and model-specific repair notes.
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 435 manual (PDF)
- Vaillant ecoFIT Pure 835 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026