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 22 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.
- ecoFIT Pure 412
- ecoFIT Pure 418
- ecoFIT Pure 425
- ecoFIT Pure 435
- ecoFIT Pure 825
- ecoFIT Pure 830
- ecoFIT Pure 835
- ecoTEC exclusive Repair: £0-120
- ecoTEC plus
- ecoTEC plus VU GB 656/4-5 A
- ecoTEC pro
- ecoTEC sustain Repair: £0-80
- 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.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 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