BoilerFaultCodes Fault 50 / 121 Low
Boiler Fault Code Fault 50 / 121
Also written as FAULT50121 — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code Fault 50 / 121 appears across 1 boiler brand and 2 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.
Most common meaning Your solar heating system is unable to move water because the pump is stuck or there is an air bubble trapped inside the pipes. (As reported on Worcester Bosch Greenstar CDi Compact)
Pick your boiler brand for the Fault 50 / 121 fix
Source
Information for the Fault 50 / 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