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 27 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
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Avoid the next call-out bill
A cover plan would handle faults like S.99: no call-out fees, no surprise bills.
Affiliate links. We may earn a commission at no extra cost to you.
Vaillant models that show the S.99 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
- ecoFIT sustain 615
- ecoFIT sustain 618
- ecoFIT sustain 630
- ecoTEC exclusive 627
- ecoTEC plus 412
- ecoTEC plus 415
- ecoTEC plus 418
- ecoTEC plus 424
- ecoTEC plus 430
- ecoTEC Plus 610
- ecoTEC Plus 620
- ecoTEC Plus 625
- ecoTEC Plus 630
- ecoTEC Plus 635
- ecoTEC Plus 832
- ecoTEC Plus 840
- ecoTEC Pro
- ecoTEC Pro / ecoTEC Plus
- Home Combi
- Home System
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.
- Vaillant ecoFIT Pure 425 manual (PDF)
- Vaillant ecoFIT Pure 835 manual (PDF)
- Vaillant ecoFIT sustain 630 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026