BoilerFaultCodes A4 Medium
Boiler Fault Code A4
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code A4 appears across 2 boiler brands and 7 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.
Most common meaning Your boiler's hot water sensor is struggling to communicate with the main control board, meaning the system cannot accurately monitor the temperature of your hot water. (As reported on Worcester Bosch Highflow 550 CDi)
Pick your boiler brand for the A4 fix
Source
Information for the A4 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