Diagnosis
The paired sensor is not assigned to an active Comfort Setting (Home, Away, Sleep) or is assigned incorrectly.
How to Fix It
Go to Menu > Comfort Control. Select a Comfort Setting (e.g., Home). Select Participation. You will see the thermostat and all paired sensors. Ensure the sensor you want to use is checked (highlighted) for that specific Comfort Setting. Repeat for each Comfort Setting as needed.
If That Didn't Work
The sensor may have a weak signal. Check its battery level in Menu > Sensors. Move the sensor closer to the thermostat (within 25 feet, line-of-sight) to re-establish a strong RF link, then move it back to its desired location.
Heads Up
The thermostat uses the average temperature of all participating devices in a Comfort Setting. If both the thermostat and a sensor are participating, it will average the two readings.