honeywell t9 wireless sensor won't connect to the thermostat, what's the trick?

Yeah, the pairing process is specific. You need to put the sensor into discovery mode first.

Here's the fix:

1. Go to the wireless sensor itself. Press and hold the button on it for 15 seconds. You'll know it's ready when the LED turns amber.
2. Now, go back to your thermostat. Press the Menu button, then navigate to Devices and Sensors (or just 'Sensors' in the menu).
3. Follow the on-screen instructions on the thermostat to add the sensor.

IF THAT DIDN'T WORK: Range or interference can be a problem. The sensor has a theoretical 200-foot range, but walls and other electronics can cut that down. Try pairing with the sensor in the same room as the thermostat, then move it to its final location.

HEADS UP: The thermostat can support up to 20 sensors for temperature, humidity, and occupancy detection.