40 Medium

Boiler Fault Code 40

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed

Fault code 40 appears across 23 boiler brands and 76 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-F)

Pick your boiler brand for the 40 fix

Atag 40 8 models Baxi 40 18 models Biasi 40 4 models British Gas 40 1 model Chaffoteaux 40 1 model Daikin 40 1 model ECA 40 1 model Ferroli 40 4 models Firebird 40 1 model Grant 40 1 model Heatline 40 3 models Ideal 40 1 model Johnson Starley 40 1 model Main 40 5 models Malvern 40 1 model Navien 40 1 model Potterton 40 5 models Saunier Duval 40 1 model Servowarm 40 1 model Viessmann 40 9 models Vokera 40 5 models Warmworld 40 1 model Worcester Bosch 40 2 models

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 7 July 2026