Q. Why is my tire leaking when I thread on the sensor to my valve stem? 

A.  This is due to the Schrader valve not being fully seated or sticking out above the lip of the valve stem. Make sure the Schrader valve (push valve) in the valve stem is recessed and doesn't stick out above the lip of the valve stem. Failure to do so can lead to air leak if the sensor can't be threaded down all the way to make a good seal.