Medium
A9 on Viessmann Vitodens 111-W

Your boiler is having trouble 'talking' to your thermostat, which means it might not know when to turn your heating on or off.

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

Control mode. Internal fault during communication with the OpenTherm room thermostat.

From this model's manual
Verified against the Viessmann Vitodens 111 W installation & service manual, page 56.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

What causes this fault?

This fault occurs when the boiler's main control board loses its digital connection with the room thermostat. The most common reason is a loose or damaged connection wire, though it can also be caused by a software glitch or a hardware failure within the thermostat itself.

DIY fix possible

Check that your thermostat is powered on and has fresh batteries if applicable.Restart the boiler using the power switch or reset button to refresh the connection.Ensure the thermostat wiring is securely plugged into its base if it is a wall-mounted unit.

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