BoilerFaultCodesVaillant S.O Fault Code
Also written as SO — same fault.
The Vaillant S.O fault code means: your boiler is working correctly but is currently in standby because it hasn't received a signal from your thermostat to turn the heating on.. It appears on 4 Vaillant models in our database.
Technical description: Heating issue, no heat demand
What causes the Vaillant S.O fault?
This status occurs when the boiler is powered on and ready to work, but your internal controls have not told it to start heating. The most common reason is that your room thermostat is set lower than the current room temperature or the programmer/timer is currently in an 'off' period. It can also signify that the wireless connection between your thermostat and the boiler has been lost.
This is very common during the transition into autumn, when homeowners first try to use their heating but find the 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 heating programmer or app is set to an 'On' period.Replace the batteries in your wireless thermostat if the screen is blank or showing a low battery symbol.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Vaillant models that show the S.O fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant S.O fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026