BoilerFaultCodesViessmann 91:1 Fault Code
Also written as 911 — same fault.
The Viessmann 91:1 fault code means: 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.. It appears on 5 Viessmann models in our database.
Technical description: External heating program changeover applies to heating circuit without mixer
What causes the Viessmann 91:1 fault?
This fault occurs when an external control device, such as a smart thermostat or timer, sends a signal to change the heating program for a specific part of your system that isn't set up to understand that command. The most common reason is a configuration error in the boiler's software where it thinks it should be controlling a specialized mixing valve that doesn't actually exist. It can also be triggered by a loose wire or a faulty connection between your external room thermostat and the boiler's internal control board.
DIY fix possible
Check any external heating controllers or smart thermostats for conflicting schedule settings.Perform a simple reset of the boiler using the power button to clear the communication glitch.Verify if any external 'party mode' or 'holiday mode' switches have been accidentally toggled.
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Gas Safe Register →Viessmann models that show the 91:1 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 91:1 fault code information on this page is sourced and cross-checked against the official Viessmann service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026