BoilerFaultCodes A3 High
Boiler Fault Code A3
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code A3 appears across 2 boiler brands and 6 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.
Most common meaning Your boiler has switched itself off because a sensor inside detects that the exhaust system or internal temperature isn't configured exactly as it should be. (As reported on Viessmann Vitodens 100-W)
Pick your boiler brand for the A3 fix
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 27 April 2026