To factory reset a Room Sensor:
- Remove the battery cover from the Room Sensor (see this link for how to open the room sensor and how to change the battery).
- Press the small switch at the top left of the circuit board 4 times within 1.5 seconds, and on the last press keep the switch pressed down.
- The red LED will illuminate and then extinguish.
- As soon as the LED turns off, release the button.
The Room Sensor is now in a factory reset default state, to add it back onto the system again see this link.
Here is a link to a help page which gives trouble shooting advice about the Room Sensor in general.