BoilerFaultCodesVaillant S.39 Fault Code
Also written as S39 — same fault.
The Vaillant S.39 fault code means: your boiler has paused because it detected that the water temperature is too high for your underfloor heating system, triggered by a safety safety switch to protect your floors.. It appears on 29 Vaillant models in our database.
Technical description: Contact thermostat has been activated (under floor systems)
What causes the Vaillant S.39 fault?
This fault occurs when an external safety sensor detects that the water flowing into your underfloor heating pipes has exceeded a safe temperature. It is usually triggered by a failure in the mixing valve or the underfloor pump, which prevents the hot boiler water from being cooled down properly before entering the floor. This safety measure is essential to prevent high temperatures from damaging your flooring or the pipes underneath.
This issue is most common during the first cold spells of autumn/winter when underfloor heating systems are turned on for the first time after being idle all summer.
DIY fix possible
Wait for the system to cool down naturally for 30-60 minutes.Check that the underfloor heating pump is running and hasn't seized.Ensure the mixing valve for the underfloor heating is not set too high.
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Vaillant models that show the S.39 fault code
Click a model for the full fault code list and model-specific repair notes.
- ecoFIT Pure 412 Repair: £120-250
- ecoFIT Pure 418 Repair: £120-250
- ecoFIT Pure 425 Repair: £120-250
- ecoFIT Pure 435 Repair: £100-250
- ecoFIT Pure 825 Repair: £100-250
- ecoFIT Pure 830 Repair: £120-250
- ecoFIT Pure 835 Repair: £120-250
- ecoMAX Repair: £120-250
- ecoMAX 613 Repair: £0-120
- ecoMAX 618 Repair: £0-120
- ecoMAX 622 Repair: £0-120
- ecoMAX 635 Repair: £0-120
- ecoMAX 824/2 E Repair: £0-120
- ecoMAX 828/2 E Repair: £0-120
- ecoMAX 835 E Repair: £0-120
- Ecomax Condensing 600/800 series Repair: £120-250
- ecoTEC plus VU GB 656/4-5 A Repair: £120-250
- ecoTEC pro Repair: £100-250
- ecoTEC sustain Repair: £80-180
- Home Combi 25 Repair: £120-250
- Home Combi 30 Repair: £120-250
- Home Combi 35 Repair: £120-250
- Turbomax 824/828 Repair: £120-250
- Turbomax Plus Repair: £120-250
- Turbomax Plus 824/828E Repair: £120-250
- Turbomax Plus 828 Repair: £90-180
- Turbomax Plus 828 E Repair: £120-250
- Turbomax Plus VUW 242E Repair: £120-250
- Turbomax Plus VUW 242EH Repair: £120-250
Source
Vaillant S.39 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