BoilerFaultCodesVaillant S.93 Fault Code
Also written as S93 — same fault.
The Vaillant S.93 fault code means: your boiler is struggling to accurately check the quality of its exhaust gases, which is preventing it from completing its ignition cycle safely.. It appears on 4 Vaillant models in our database.
Technical description: Flue gas measurement not possible. Flue gas analysis is not currently possible.
What causes the Vaillant S.93 fault?
This fault occurs when the boiler's internal sensors fail to accurately measure the combustion gases during the startup phase. It is often caused by a faulty gas valve or worn electrodes failing to produce a stable flame that the system can verify. Without a reliable reading of these exhaust gases, the boiler enters a safety mode to prevent inefficient or dangerous operation.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Vaillant models that show the S.93 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant S.93 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