A9 High

Boiler Fault Code A9

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed

Fault code A9 appears across 3 boiler brands and 10 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler is having trouble 'talking' to its internal pump, which means it cannot flow water around the system to heat your home. (As reported on Viessmann Vitodens)

Pick your boiler brand for the A9 fix

Atag A9 2 models Viessmann A9 7 models Worcester Bosch A9 1 model

Source

Information for the A9 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