BoilerFaultCodesVaillant S.0 Fault Code
Also written as S0 — same fault.
The Vaillant S.0 fault code means: your boiler is working correctly but is currently idle because it hasn't been told to turn on by your thermostat or timer.. It appears on 19 Vaillant models in our database.
Technical description: No heat demand (heating operation)
What causes the Vaillant S.0 fault?
This code appears when the boiler system is functioning correctly but is waiting for a command from your external controls. It physically occurs because the electrical circuit between your thermostat or programmer and the boiler is open, signaling that your home is already at the desired temperature. The most common reasons for this staying on screen when you want heat are depleted thermostat batteries or a programmer set to the 'off' position.
This status is frequently seen during the transition into autumn when homeowners first try to turn their heating back on, only to find their thermostat batteries have died over the summer.
DIY fix possible
Check that your room thermostat is turned up higher than the current room temperature.Ensure your central heating programmer or timer is set to an 'ON' period.Check if the batteries in your wireless thermostat need replacing.
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Vaillant models that show the S.0 fault code
Click a model for the full fault code list and model-specific repair notes.
- ecoFIT Pure 412
- ecoFIT Pure 435
- 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 VU GB 656/4-5 A
- Home Combi 25
- Home Combi 30
- Home Combi 35
Source
Vaillant S.0 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 435 manual (PDF)
- Vaillant ecoFIT Pure 835 manual (PDF)
- Vaillant Ecomax Condensing 600/800 series manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026