21 High

Boiler Fault Code 21

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed

Fault code 21 appears across 12 boiler brands and 40 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 Baxi 21 1 model Biasi 21 4 models Ferroli 21 3 models Glow-worm 21 1 model Ideal 21 1 model Intergas 21 10 models Johnson Starley 21 10 models Main 21 1 model Potterton 21 1 model Saunier Duval 21 4 models Vokera 21 3 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 27 April 2026