Wondering if some one can tell me if the sensors need to be under pressure for the vehicle to recognize them. I bought a set of winter tires and rims for the girlfriends car but they did not come with sensors. I ordered a set of sensors off Amazon. This morning I tried to learn the sensors to the vehicle before installing them in the tires ( just to make sure they work). I could not get them to learn. Just to double check I used my tool to learn the summer tires and it worked with no issues. Not sure if I got dead sensors or the wrong ones or if they need to be in the tire under pressure to work. Any help is greatly appreciated.