91:2 Low

Viessmann 91:2 Fault Code

Also written as 912 — same fault.

The Viessmann 91:2 fault code means: your boiler is experiencing a communication issue with the accessory that controls different temperature zones in your home, often caused by a temporary wiring hitch or a sensor problem.. It appears on 5 Viessmann models in our database.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description: External heating program changeover applies to heating circuit with mixer

What causes the Viessmann 91:2 fault?

This fault occurs when the boiler’s main control board loses the electronic signal from the module that manages different heating zones or underfloor heating. It is usually caused by a loose connection in the wiring between these components or a fault within the external mixer module itself, which prevents the boiler from knowing which temperature to send to specific parts of your home.

This issue is often noticed in autumn when homeowners switch their heating back on after summer and the external control modules fail to synchronise correctly.

Typical repair cost £90-200 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Viessmann models that show the 91:2 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Viessmann 91:2 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