Medium
C6 on Viessmann Vitodens 300-W

Your boiler is having trouble communicating with the internal pump that moves water around your second heating zone, which may prevent some of your radiators from getting hot.

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

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

Engineer required

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

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Related fault codes

C5View →C7View →F9View →

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