A3 High

Boiler Fault Code A3

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed

Fault code A3 appears across 5 boiler brands and 13 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 that a temperature probe inside the flue system has shifted from its correct position and needs to be physically reset. (As reported on Viessmann Vitodens 242-F B2UA)

Pick your boiler brand for the A3 fix

Ideal A3 1 model Sime A3 1 model Viessmann A3 1 model Vokera A3 1 model Worcester Bosch A3 9 models

Source

Information for the A3 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 31 August 2026