BoilerFaultCodes 233 / 9U High
Boiler Fault Code 233 / 9U
Also written as 2339U — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code 233 / 9U 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's main control unit is failing to communicate properly, which has caused the system to shut down for safety. (As reported on Worcester Bosch Greenstar CDi Compact)
Pick your boiler brand for the 233 / 9U fix
Source
Information for the 233 / 9U 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