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.

What the manual says

Manufacturer manual

Communication fault-ext. extension coding address 2E settings

From this model's manual
Verified against the Viessmann Vitodens 200 W installation & service manual, page 96.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

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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