T9/T10 Thermostat Error Messages: “Indoor Sensor Connection Error” and “Error Adding Device – The device could not be added, please try again”

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.