BoilerFaultCodes 21 High
Boiler Fault Code 21
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code 21 appears across 20 boiler brands and 60 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.
Most common meaning Your boiler has shut down because a safety sensor has detected that the burner area is getting too hot or is failing to register a secure connection. (As reported on Ideal Maximiser SE)
Pick your boiler brand for the 21 fix
Alpha 21 1 model Atmos 21 4 models Baxi 21 1 model Biasi 21 4 models Daikin 21 3 models Ferroli 21 4 models Gledhill 21 1 model Glow-worm 21 3 models Heatline 21 3 models Ideal 21 2 models Intergas 21 11 models Johnson Starley 21 5 models Main 21 2 models Potterton 21 1 model Saunier Duval 21 4 models Sime 21 2 models Unical 21 2 models Vaillant 21 1 model Vokera 21 4 models Worcester Bosch 21 2 models
Source
Information for the 21 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