Display shows 'Err' without a numeric code and thermostat does not respond to inputs.

Safety: If mains powered, isolate power before removing the thermostat from its base.

• Power reset: Remove the thermostat from the wall plate or remove batteries for at least 2 minutes. Reinstall and check if 'Err' clears.
• Factory reset: Use the reset button or menu sequence described in the Eurotronic manual to restore factory settings. Reprogram schedules and setpoints.
• Check buttons/knob: Ensure no button is stuck or the rotary knob is not jammed. Gently press each button to confirm free movement.
• Check environment: Excessive moisture or condensation can cause temporary errors. Allow the unit to dry in a normal indoor environment.
• Replace thermostat: If 'Err' reappears frequently, the PCB or firmware is likely defective and the thermostat should be replaced.