Medium Warning
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

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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

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

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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