BoilerFaultCodes 4-b5 Low
Boiler Fault Code 4-b5
Also written as 4B5 — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code 4-b5 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 notifying you that its internal protection against freezing has been activated or deactivated, which is a normal safety monitoring function. (As reported on Worcester Bosch Greenstar 2000)
Pick your boiler brand for the 4-b5 fix
Source
Information for the 4-b5 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