Thermostat shows A9 and cannot read floor temperature in underfloor heating mode; floor protection features are disabled.

Safety first: Turn off power to the thermostat circuit before removing it from the wall.

• Verify sensor connection: Remove the thermostat from its base and check that the floor sensor wires are firmly connected to the designated sensor terminals.
• Inspect cable route: If accessible, inspect the floor sensor cable from the thermostat to the floor entry point for cuts or pinches.
• Check configuration: In the thermostat settings, ensure that an external floor sensor is enabled and that the correct sensor type (resistance curve) is selected according to the Viessmann documentation.
• Measure sensor resistance: If you have a multimeter and are competent, measure the resistance of the floor sensor at room temperature and compare it to the values in the manual. Infinite or zero resistance indicates a broken or shorted sensor.
• Replace sensor if faulty: Install a new Viessmann‑compatible floor temperature sensor if the existing one is defective.