First, reset the sensor by pressing and holding the Connect button on the sensor for about 15 seconds, then, with the sensor within 2 feet of the thermostat, try adding it again. Next, if you’re still having issues, replace the sensor batteries and retry. Finally, try removing the batteries for 30 seconds and re-inserting them to clear the error. Look for large appliances or metal and concrete walls that may reduce the signal strength. Place the sensor within the least-obstructed, straight-line path to the thermostat. Then, try adding the sensor to the thermostat.