BoilerFaultCodes A21 High
Boiler Fault Code A21
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code A21 appears across 1 boiler brand and 3 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 that water is not circulating correctly through the system, preventing it from firing up to provide heat or hot water. (As reported on Ariston Genus 27)
Pick your boiler brand for the A21 fix
Source
Information for the A21 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