Medium
bA on Viessmann Vitodens 300-W

Your boiler has lost communication with the control box that manages your second heating zone, meaning your underfloor heating or additional radiators may not warm up 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

Mixer M2 continues to regulate: Communication fault - extension kit for mixer 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

b7View →bcView →bdView →

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