Medium
CE on Viessmann Vitodens 300-W

Your boiler is having trouble 'talking' to an external accessory, such as an extra heating controller or a smart home module.

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-ext. extension

DIY fix possible

Locate the external control module or thermostat receiverCheck for any loose cables or plugs connecting external components to the boilerTurn the boiler power off and back on to attempt to reset the communication link

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Related fault codes

C5View →C6View →CFView →

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