The sensors are inside the tire/wheel at the valvestem. The sensor sends its signal via radio waves, just like the ones you can buy as add-ons. There is no procedure that needs be done when dismounting/mounting tires (except not to break the sensor). No telling which one is reading bad, and I don't know an easy way of testing each sensor. Might try inflating each tire to 60 psi or so, one at a time to see if the light will go out. After overinflating each tire, I think you will have to turn the igniton key off and back on to reset the light once it has illuminated. And 60 psi won't hurt the tire, just don't drive the car with the tires overinflated. You might also jack each wheel up and spin it to see if the sensor was knocked loose or something.