BoilerFaultCodesViessmann C6 Fault Code
The Viessmann C6 fault code means: your boiler is having trouble communicating with the internal pump that moves hot water around your heating system.. It appears on 8 Viessmann models in our database.
Technical description: Control mode, max. pump speed: Communication fault - variable speed heating circuit pump, heating circuit M2
What causes the Viessmann C6 fault?
This fault occurs when the boiler's main control board loses its digital connection with the internal pump, preventing the system from synchronising water flow with heat demand. The most common reason is a failure in the pump's internal electronic module or a loose connection in the low-voltage communication data cable. In some cases, a power surge or moisture ingress can damage the circuitry responsible for these signals.
This fault is frequently seen at the start of autumn when a pump that has remained stationary all summer is suddenly required to ramp up to full speed.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Viessmann models that show the C6 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann C6 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