BoilerFaultCodesViessmann C2 Fault Code
The Viessmann C2 fault code means: your boiler is having trouble 'talking' to your solar panels or an additional heating control part, which might prevent those specific systems from working correctly.. It appears on 7 Viessmann models in our database.
Technical description: Communication fault: Solar control module or extension module
What causes the Viessmann C2 fault?
This fault occurs when the boiler's main computer loses its digital connection with the supplementary units that manage your solar thermal panels or extra heating zones. It is usually caused by a loose wiring connection or an internal electronic failure within the expansion module itself, meaning the boiler can no longer send or receive instructions from those parts of your system.
DIY fix possible
Locate the power switch (fused spur) for the boiler.Turn the power off, wait for 30 seconds, and turn it back on to see if the connection resets.Check if any external solar controllers or expansion boxes have lost power or been unplugged.
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Gas Safe Register →Viessmann models that show the C2 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann C2 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