If there are multiple tado° devices installed in one zone, they synchronize their settings. If you change the temperature on a Smart Thermostat or Smart Radiator Thermostat, all devices in the same zone will receive the change. The same applies for an Extension Kit installed alongside one or multiple Smart Thermostats or Smart Radiator Thermostats.
However, sometimes changes cannot be transmitted immediately, for instance due to a temporary loss of radio link. The lack of synchronization between devices is indicated by the sync error. On the Smart Thermostat the error is displayed by two arrows and on the Smart Radiator Thermostats by a rotating circle.
While the symbol is shown, the device keeps sending the changes to other devices in the same zone until they are received. Any further change on the same or a different device during that time overrides the previous one.
The most common reason for non-transmitted changes is a temporary loss of radio link of any of the devices to the Internet Bridge, which is usually resolved by itself within a few minutes. However, if the interruption persists for more than 30 minutes, it is recommended to reset the devices.