BoilerFaultCodesVaillant S.17 Fault Code
Also written as S17 — same fault.
The Vaillant S.17 fault code means: your boiler is simply performing a routine cooling-down process after you have finished using hot water, so there is no need for concern.. It appears on 26 Vaillant models in our database.
Technical description: DHWmode:Pumpoverrun
What causes the Vaillant S.17 fault?
This fault occurs when the boiler's sensors detect that water is returning to the unit too quickly without losing heat, which usually means the water isn't circulating through your radiators properly. The most common reason is that the valves on your radiators are closed or there is a physical blockage in the pipework preventing the flow. It can also be triggered if the boiler's internal pump is struggling to push water around the system.
This code often appears at the end of autumn when heating systems are switched back on after being idle for several months, which can cause internal parts or valves to seize up.
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.17 fault code
Click a model for the full fault code list and model-specific repair notes.
- ecoFIT Pure 825
- ecoFIT Pure 830
- ecoFIT Pure 835
- ecoMAX
- ecoMAX 613 Repair: £0-80
- ecoMAX 618 Repair: £0-80
- ecoMAX 622 Repair: £0-80
- ecoMAX 635 Repair: £0-80
- ecoMAX 824/2 E Repair: £0-80
- ecoMAX 828/2 E Repair: £0-80
- ecoMAX 835 E Repair: £0-80
- Ecomax Condensing 600/800 series
- ecoTEC exclusive
- ecoTEC plus
- ecoTEC pro
- ecoTEC sustain
- Home Combi 25
- Home Combi 30
- Home Combi 35
- Turbomax 824/828
- Turbomax Plus
- Turbomax Plus 824/828E
- Turbomax Plus 828
- Turbomax Plus 828 E
- Turbomax Plus VUW 242E
- Turbomax Plus VUW 242EH
Source
Vaillant S.17 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 835 manual (PDF)
- Vaillant Ecomax Condensing 600/800 series manual (PDF)
- Vaillant ecoMAX 835 E manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026