C1 Medium

Viessmann C1 Fault Code

The Viessmann C1 fault code means: your boiler has lost the digital connection to an external accessory, such as a solar thermal unit or an additional heating control, preventing the system from coordinating properly.. It appears on 2 Viessmann models in our database.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: Communication error, extension EA1
Typical repair cost £150-300 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Viessmann models that show the C1 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Viessmann C1 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