A9 Medium

Viessmann A9 Fault Code

The Viessmann A9 fault code means: your boiler is having trouble 'talking' to your thermostat, which means it might not know when to turn your heating on or off.. It appears on 9 Viessmann models in our database.

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.

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

What causes the Viessmann A9 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.

Typical repair cost £90-180 (including labour)

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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Viessmann models that show the A9 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Viessmann A9 fault code information on this page is sourced and cross-checked against the official Viessmann service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026