40 Medium

Boiler Fault Code 40

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed

Fault code 40 appears across 12 boiler brands and 38 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler has detected a faulty temperature sensor in one of your heating zones, which is preventing that specific circuit from getting warm. (As reported on Viessmann Vitodens 333)

Pick your boiler brand for the 40 fix

Atag 40 4 models Biasi 40 4 models Chaffoteaux 40 2 models Ferroli 40 3 models Grant 40 1 model Ideal 40 1 model Johnson Starley 40 6 models Main 40 1 model Saunier Duval 40 2 models Viessmann 40 9 models Vokera 40 4 models Worcester Bosch 40 1 model

Source

Information for the 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