Medium Warning
bA on Viessmann Vitodens 222-W

Your boiler is having trouble communicating with the control box that manages your second heating zone, meaning that specific area may not heat up correctly.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Book a service

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

What causes this fault?

This fault occurs when the boiler's main brain loses its connection with the electronic module that controls your underfloor heating or second radiator zone. It is usually caused by a loose wiring connection or a component failure within the external control box that manages that specific heating circuit.

This issue often comes to light during the first cold spell of autumn when homeowners try to turn on a secondary heating zone that has been inactive all 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

b7View →bcView →bdView →beView →

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