BoilerFaultCodesViessmann C5 Fault Code
The Viessmann C5 fault code means: your boiler is having trouble communicating with its internal pump, which means it cannot properly move heated water around your system.. It appears on 7 Viessmann models in our database.
Technical description: Control mode, max. pump speed: Communication fault - variable speed internal pump
What causes the Viessmann C5 fault?
This fault occurs when the boiler's main control board loses its digital connection with the internal pump, preventing the system from regulating water flow. The most common reason is a failure in the pump's electronic circuitry or a loose connection in the wiring harness that carries data signals. Without this communication, the boiler cannot safely manage heat distribution and will stop operating to prevent overheating.
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 C5 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann C5 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