BoilerFaultCodes ALL 21 Medium
Boiler Fault Code ALL 21
Also written as ALL21 — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code ALL 21 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 lost the ability to accurately measure the temperature of the water being sent to your radiators, which may cause your heating system to operate inefficiently or stop working in specific zones. (As reported on Sime Murelle Elite HE Combi)
Pick your boiler brand for the ALL 21 fix
Source
Information for the ALL 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