BoilerFaultCodes 121 High
Boiler Fault Code 121
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code 121 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 that water is not flowing correctly through the system to heat your home or water effectively. (As reported on Ferroli Sigma 40 FF)
Pick your boiler brand for the 121 fix
Source
Information for the 121 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