Medium
A9 on Viessmann Vitodens 100-W WB1B

Your boiler is having a temporary communication breakdown with your external room thermostat or smart control system, which may prevent it from firing 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.

What the manual says

Manufacturer manual

Communication fault Open Therm device

From this model's manual
Verified against the Viessmann Vitodens 100 W Wb1b System installation & service manual, page 34.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

Engineer required

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

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