BoilerFaultCodesViessmann 91:3 Fault Code
Also written as 913 — same fault.
The Viessmann 91:3 fault code means: your boiler is receiving conflicting signals about how to heat your home because an external controller is trying to switch settings for several heating zones at once.. It appears on 5 Viessmann models in our database.
Technical description: External heating program changeover applies to heating circuit without mixer and heating circuit with mixer
What causes the Viessmann 91:3 fault?
This fault occurs when the boiler receives two different instructions at the same time from your external heating controls, such as a wall thermostat or a zone controller. It essentially creates a logic 'argument' within the system because the boiler doesn't know which setting to prioritize for your radiators. The most common reason is a wiring error or a faulty external programmer sending overlapping signals to the control board.
DIY fix possible
Check any external smart thermostats or zone controllers for low batteries or error messagesEnsure all external timers and programmers are set to the same heating modePerform a simple reset of the boiler using the power button to clear the communication glitch
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Gas Safe Register →Viessmann models that show the 91:3 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 91:3 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