BoilerFaultCodesVaillant S.25 Fault Code
Also written as S25 — same fault.
The Vaillant S.25 fault code means: your boiler is currently just finishing its standard cooling-down process after heating your water and is not experiencing a fault.. It appears on 17 Vaillant models in our database.
Technical description: DHWmode:Pump/fanoverrun
What causes the Vaillant S.25 fault?
This status occurs when the boiler's internal computer is verifying that the pump is moving enough water and the fan is spinning at the right speed before ignition. It is essentially a safety check to ensure the heat exchanger won't overheat once the flame starts. If the display stays on this code for a long time, it usually means the pump is struggling to move water or a sensor is providing an inaccurate reading.
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.25 fault code
Click a model for the full fault code list and model-specific repair notes.
- ecoFIT Pure 412
- ecoFIT Pure 418
- ecoFIT Pure 425
- ecoFIT Pure 435
- ecoFIT Pure 825
- ecoFIT Pure 830
- ecoFIT Pure 835
- ecoMAX Repair: £120-250
- Ecomax Condensing 600/800 series Repair: £120-250
- ecoTEC exclusive
- ecoTEC plus
- ecoTEC plus VU GB 656/4-5 A
- ecoTEC pro
- ecoTEC sustain
- Home Combi 25
- Home Combi 30
- Home Combi 35
Source
Vaillant S.25 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