S.42 High

Vaillant S.42 Fault Code

Also written as S42 — same fault.

The Vaillant S.42 fault code means: your boiler has stopped working because its internal safety system detected that wastewater cannot be pumped away or an add-on component is unresponsive.. It appears on 26 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: Response from accessory module or defective condensate pump is blocking burner operation

What causes the Vaillant S.42 fault?

This fault occurs when the boiler is unable to safely discharge the acidic water produced during the heating process. It is typically triggered because an external pump used to move this water has failed, or the internal electronic module that communicates with that pump has developed a fault. To prevent water from backing up and damaging the internal components, the boiler automatically shuts down as a safety precaution.

This fault is more common during freezing winter temperatures, as the external pipework can freeze and cause the pump to overflow or fail.

Typical repair cost £120-280 (including labour)

DIY fix possible

Check if the external condensate pipe is frozen outside your homeInspect the condensate pump's power cable to ensure it is plugged inCarefully check if the wastewater pipe or drain is blocked

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Vaillant models that show the S.42 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Vaillant S.42 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