Safety first: Do not use excessive force on the screen or buttons.

Clean surface: Gently clean the screen or buttons with a soft, slightly damp cloth. Do not use solvents.
Reboot device: Power cycle the thermostat by removing power or batteries for 1–2 minutes.
Use app as workaround: Control the system via the Viessmann app if the physical interface is unreliable.
Replace thermostat: If input remains unresponsive, the UI hardware is likely defective and the thermostat should be replaced.

Thermostat buttons or touchscreen are unresponsive or laggy.

Possible Causes