When you see the LEARN L FRONT TIRE message, go to the left front tire and prepare to recalibrate the sensor. GM calls for tool number J 41760 to be placed on the valve stem. You can do the same thing with a common magnet like what is typically used on a magnetic pick-up tool. Chris simply works the magnet around the valve stem until the horn sounds, which indicates that the sensor is calibrated. Then calibrate the next Tire Pressure Sensor as directed by the DIC. The order should be as follows: left front, right front, right rear, left rear.