BoilerFaultCodesVaillant S.99 Fault Code
Also written as S99 — same fault.
The Vaillant S.99 fault code means: your boiler is simply performing a routine internal check-up to ensure everything is working correctly and will resume normal operation shortly.. It appears on 18 Vaillant models in our database.
Technical description: Vaillant self-test. The Vaillant self-test is activated.
What causes the Vaillant S.99 fault?
This code appears when the boiler’s internal computer runs a temporary diagnostic check to verify that all sensors and safety components are communicating correctly. It is not triggered by a broken part, but rather a routine automated sequence that briefly pauses heating activity. Once the boiler confirms all systems are healthy, the code will clear and the unit will fire up as normal.
DIY fix possible
Wait for up to 15 minutes for the boiler to complete its automatic testMonitor the display to see if it returns to a normal temperature or status codeIf the code persists for over an hour, try turning the boiler off and on again once
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Gas Safe Register →Vaillant models that show the S.99 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant S.99 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