BoilerFaultCodes AA 9999 High
Boiler Fault Code AA 9999
Also written as AA9999 — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code AA 9999 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 computer is having trouble communicating with its internal safety systems, which means it has shut down to prevent any unsafe operation. (As reported on Ariston Clas One)
Pick your boiler brand for the AA 9999 fix
Source
Information for the AA 9999 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