BoilerFaultCodes H12 Medium
Boiler Fault Code H12
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code H12 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 is struggling to detect the temperature of your hot water cylinder, which means it may fail to heat your water properly. (As reported on Worcester Bosch Greenstar 12i System)
Pick your boiler brand for the H12 fix
Source
Information for the H12 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