BoilerFaultCodes C21 High
Boiler Fault Code C21
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code C21 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 has detected a communication issue with the sensor that monitors exhaust gases and has safely shut down to prevent potential overheating or unsafe operation. (As reported on Atag Premier)
Pick your boiler brand for the C21 fix
Source
Information for the C21 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