Boiler Fault Code 91:1

Also written as 911 — same fault.

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

Fault code 91:1 appears across 1 boiler brand and 5 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 an external control conflict where a heating setting is trying to communicate with a part of the system that isn't configured to receive it. (As reported on Viessmann Vitodens 200)