BoilerFaultCodesViessmann CF Fault Code
The Viessmann CF fault code means: your boiler is having trouble 'talking' to its internal wi-fi or internet connection module, which may prevent you from controlling it via apps or external thermostats.. It appears on 1 Viessmann model in our database.
Technical description: Communications fault-LON communication module
What causes the Viessmann CF fault?
This fault occurs when the boiler's main control board loses its digital connection with the internal Wi-Fi or communication module. It is usually caused by a loose wiring plug, a software glitch in the module, or a hardware failure on the communication board itself.
DIY fix possible
Turn the boiler off at the main power switch for 30 seconds, then turn it back on to reboot the electronics.Check your home internet router is working correctly and within range of the boiler.Verify that any external smart thermostats or bridges are powered on and connected.
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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 7 July 2026