BoilerFaultCodes S.40 Low
Boiler Fault Code S.40
Also written as S40 — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code S.40 appears across 1 boiler brand and 8 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.
Most common meaning Your boiler is temporarily running at a lower power level to prevent a fault and ensure it can still provide some warmth while it detects a minor issue. (As reported on Vaillant ecoTEC Pro)
Pick your boiler brand for the S.40 fix
Source
Information for the S.40 fault code is sourced from the official service / installation manual for each model that displays it. Read more about how we source fault codes →
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026