BoilerFaultCodes A-b4 Medium
Boiler Fault Code A-b4
Also written as AB4 — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code A-b4 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 an issue with the sensor that monitors the temperature of the hot water leaving the unit, which may result in inconsistent water temperatures. (As reported on Worcester Bosch Greenstar 8000 Life/Style)
Pick your boiler brand for the A-b4 fix
Source
Information for the A-b4 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