BoilerFaultCodesViessmann 91:0 Fault Code
Also written as 910 — same fault.
The Viessmann 91:0 fault code means: your boiler is having trouble communicating with an external control accessory, meaning some remote settings or external commands might not be working as expected.. It appears on 5 Viessmann models in our database.
Technical description: No external operating mode changeover via external extension
What causes the Viessmann 91:0 fault?
This fault occurs when the boiler loses its physical connection to an external accessory, such as a plug-in extension module or a third-party controller. It is usually caused by a loose communication wire or a hardware failure within the interface module that handles external commands. While the boiler typically continues to heat, it can no longer receive specific instructions to change its operating mode from these outside devices.
DIY fix possible
Locate your boiler's power switch and turn it off for 30 seconds, then turn it back on to reset the communication.Check if any external smart home controllers or extensions are unplugged or have lost power.Confirm that your thermostat or external control unit is displaying information and has fresh batteries if applicable.
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Gas Safe Register →Viessmann models that show the 91:0 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 91:0 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