BoilerFaultCodesVaillant S.42 Fault Code
Also written as S42 — same fault.
The Vaillant S.42 fault code means: your boiler has paused operation because it cannot safely clear waste gases or discharge wastewater produced during the heating process.. It appears on 15 Vaillant models in our database.
Technical description: Fluenon-returnflapclosed | Fluenon-returnflapresponseblocks burneroperation (onlyinconjunctionwithaccessoryVR40)orcondensate pumpdefective;heatrequirementisblocked
What causes the Vaillant S.42 fault?
This fault occurs when the boiler's internal safety sensors detect that waste fluids or gases are not being cleared from the system correctly. The most common reason is a failure in the pump that removes wastewater, or a communication error with the components that manage the exhaust gases. To prevent potential flooding or unsafe operation, the boiler automatically disables the burner until the blockage or electrical fault is resolved.
This fault is more frequent during extreme winter cold, as the external pipe that carries away wastewater can freeze and cause the internal pump to fail.
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.42 fault code
Click a model for the full fault code list and model-specific repair notes.
- ecoFIT Pure 412 Repair: £150-300
- ecoFIT Pure 418 Repair: £120-250
- ecoFIT Pure 425 Repair: £120-250
- ecoFIT Pure 435 Repair: £120-250
- ecoFIT Pure 825 Repair: £120-250
- ecoFIT Pure 830 Repair: £150-300
- ecoFIT Pure 835 Repair: £120-250
- ecoTEC exclusive Repair: £120-350
- ecoTEC plus Repair: £120-250
- ecoTEC plus VU GB 656/4-5 A Repair: £150-350
- ecoTEC pro Repair: £120-280
- ecoTEC sustain Repair: £120-250
- Home Combi 25 Repair: £150-300
- Home Combi 30 Repair: £150-300
- Home Combi 35 Repair: £150-300
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.
- 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