Boiler Fault Code A xx

Also written as AXX — same fault.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026

Fault code A xx appears across 1 boiler brand 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 is currently running a routine air-removal cycle to ensure the system is clear of air bubbles, which is a normal process after a restart or refill. (As reported on Atag A200)