Medium Warning
C6 on Viessmann Vitodens 333

Your boiler is having trouble communicating with the internal pump that manages one of your heating circuits, meaning that part of your heating may not be working correctly.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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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 fault - variable speed heating circuit pump, heating circuit M2. Control mode, max. pump speed

What causes this fault?

This fault occurs when the boiler's main control board loses its digital connection with the pump responsible for your secondary heating zone. The most common reason is a physical failure of the pump's internal electronics or a loose connection in the wiring that carries signals between the components.

This fault is more frequent in autumn when the heating is first turned back on, as the pump can seize or fail after being inactive throughout the summer.

Engineer required

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

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Related fault codes

C2View →C5View →F9View →

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