BoilerFaultCodesVaillant S.108 Fault Code
Also written as S108 — same fault.
The Vaillant S.108 fault code means: your boiler is currently performing a routine self-cleaning cycle to clear any fumes from the system before it attempts to ignite.. It appears on 15 Vaillant models in our database.
Technical description: Purgingthecombustionchamber,faninoperation
What causes the Vaillant S.108 fault?
This status occurs when the boiler detects a need to clear the internal combustion area of any residual gases or debris before it can safely start the burner. It is a natural part of the boiler's safety protocol, triggered by the fan spinning at high speed to ensure a clean environment for ignition. This cycle is usually temporary and does not signify that a component has failed.
DIY fix possible
Wait for the process to complete, as this is a normal status message and not a breakdownEnsure there are no obstructions around the external flue pipe outside your homeIf the code persists for several minutes without the boiler firing up, try resetting the boiler once
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.108 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant S.108 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