BoilerFaultCodes J6-21 Medium
Boiler Fault Code J6-21
Also written as J621 — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code J6-21 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 the water coming back from your radiators is hotter than the water going out, which usually means there is a blockage or the internal sensors are confused. (As reported on Daikin D2CND)
Pick your boiler brand for the J6-21 fix
Source
Information for the J6-21 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 27 April 2026