BoilerFaultCodes 343 High
Boiler Fault Code 343
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code 343 appears across 2 boiler brands and 5 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.
Most common meaning Your boiler is detecting an unusual temperature imbalance that suggests the internal heat exchanger or circulation system isn't working as it should, so it has shut down for protection. (As reported on Worcester Bosch 24/28/35 CDi MkII)
Pick your boiler brand for the 343 fix
Source
Information for the 343 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