BoilerFaultCodesViessmann Cd Fault Code
Also written as CD — same fault.
The Viessmann Cd fault code means: your boiler has lost its connection to the remote control or internet monitoring system, so you may not be able to adjust settings via your app or external thermostat.. It appears on 7 Viessmann models in our database.
Technical description: Communication fault Vitocom 100 (KM BUS)
What causes the Viessmann Cd fault?
This fault occurs when the boiler's main control board loses its digital handshake with the communication module responsible for internet connectivity. It is usually caused by a loose connection cable, a temporary glitch in the module's hardware, or a complete failure of the Vitocom unit itself. While the boiler will continue to provide heat based on its last known settings, it can no longer receive new instructions from your smartphone or remote thermostat.
DIY fix possible
Check that your internet router is switched on and workingEnsure the Vitocom 100 communication module is firmly plugged into the boilerTurn the boiler power off and back on again to restart the communication link
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Gas Safe Register →Viessmann models that show the Cd fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann Cd 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