BoilerFaultCodesViessmann CF Fault Code
The Viessmann CF fault code means: your boiler's internal communication system has failed, meaning the different electronic parts are no longer talking to each other correctly.. It appears on 7 Viessmann models in our database.
Technical description: Communications fault - LON communication module. Control mode
What causes the Viessmann CF fault?
This fault occurs when the main control board loses its connection with the internal plug-in communication module. The most common reason is an electronic component failure within the module itself or a loose physical connection caused by vibrations over time. Without this link, the boiler's brain cannot send or receive the data it needs to operate safely, causing it to stop running.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Viessmann models that show the CF fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann CF 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