Weather Stations sold on amazon come with its own sensor you do not need to buy a sensor. If you need an extra sensor then you would want to make the purchase.

Many modern homes have up-to-date clocks or mini weather stations that depend on radio-transmitted data to stay calibrated and current. The La Crosse TX6U works outdoors by sensing temperature and transmitting that information to an indoor station. It can transmit to a base up to 80 feet away. The TX6U can be added to augment information coming to La Crosse units already in operation. Remove batteries from base units first and activate sensors; then replace batteries in base units and press no buttons until a display appears. Multiple sensors can be assigned a number so that it will be clear which information from which sensor is displayed.

The TX6U operates on two AA batteries. Its housing is made of durable, neutral-tone plastic. For options in wall mounting, two screws and screw anchors and a strong adhesive tape are included. The TX6U measures 5-1/2 by 1-1/2 by 1 inch. La Crosse provides a one-year limited warranty on its sensors. Each sensor comes with a detailed instruction sheet. --Garland Withers