Viessmann · Vitodens 100-W Boiler
The burner is in a fault state because of a communication error on the CAN bus. This indicates a problem with how different components are communicating.
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Check the fan unit for correct function; observe it during a mains ON reference run.
Visually inspect all plug-in connections and cables of the CAN bus for firm seating and damage.
If the fault still persists, contact a qualified heating engineer, as the fan unit or other CAN bus components may need replacement.