BoilerFaultCodesVaillant S.31 Fault Code
Also written as S31 — same fault.
The Vaillant S.31 fault code means: your boiler is currently set to its summer setting or is being told by your thermostat not to turn on the heating because it thinks the house is warm enough.. It appears on 37 Vaillant models in our database.
Technical description: No heat demand: Summer mode (Summer mode active or no heat requirement from eBUS control)
What causes the Vaillant S.31 fault?
This code appears when the boiler has been instructed to disable the central heating, either by a setting you have chosen or by your external thermostat. It typically happens because the programmed schedule has reached an 'off' period or the thermostat detects that your home has already reached the desired temperature. In some cases, it may indicate that the communication signal between your wireless controller and the boiler has been interrupted.
This is very common during the transition to spring and summer when homeowners manually switch to 'Summer Mode' to save energy by only heating their hot water.
DIY fix possible
Check that the heating temperature dial or digital setting on the boiler is turned up.Check your room thermostat or smart phone app to ensure the heating is switched 'On' and the target temperature is higher than the current room temperature.Check if 'Summer Mode' is enabled on the boiler menu and deactivate it if you require heating.
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Vaillant models that show the S.31 fault code
Click a model for the full fault code list and model-specific repair notes.
- Atmomax
- ecoFIT Pure 412
- ecoFIT Pure 418
- ecoFIT Pure 425
- ecoFIT Pure 435
- ecoFIT Pure 825
- ecoFIT Pure 830
- ecoFIT Pure 835
- ecoTEC
- ecoTEC 65kW
- ecoTEC exclusive Repair: £0-120
- ecoTEC Plus 610
- ecoTEC Plus 615 Repair: £0-80
- ecoTEC Plus 620 Repair: £0-80
- ecoTEC Plus 625 Repair: £0-80
- ecoTEC Plus 630 Repair: £0-80
- ecoTEC Plus 635 Repair: £0-80
- ecoTEC Plus 826
- ecoTEC Plus 840 Repair: £0-80
- ecoTEC Plus 938 Repair: £0-80
- ecoTEC plus aquaPLUS
- ecoTEC Pro
- ecoTEC Pro / ecoTEC Plus
- ecoTEC Pro 24
- ecoTEC Pro 28
- ecoTEC Pro 30
- ecoTEC sustain Repair: £0-80
- Home Combi
- Home System
- Thermocompact
- 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.31 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 412 manual (PDF)
- Vaillant ecoFIT Pure 835 manual (PDF)
- Vaillant ecoTEC manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026