FA9 Medium

Viessmann FA9 Fault Code

The Viessmann FA9 fault code means: your boiler has lost its connection to your smart thermostat or external heating controller.. It appears on 1 Viessmann model 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: OpenTherm remote control not connected
Typical repair cost £60-140 (including labour)

DIY fix possible

Check if your thermostat batteries need replacingEnsure your internet router and hub are switched on and connectedCheck for loose wires at the thermostat base unit if it is easily accessiblePress the reset button on the boiler control panel

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Viessmann models that show the FA9 fault code

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

Source

Viessmann FA9 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