BoilerFaultCodes 33.S Low
Boiler Fault Code 33.S
Also written as 33S — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code 33.S appears across 1 boiler brand and 1 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.
Most common meaning Your boiler is currently running a routine internal systems check, which means it is communicating with its sensors to ensure everything is operating correctly. (As reported on Vaillant Thermocompact)
Pick your boiler brand for the 33.S fix
Source
Information for the 33.S 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 7 July 2026