Remember you are setting the sensors back to 'zero' so all tires must be properly inflated otherwise the sensors will not be calibrated properly and their readings will be incorrect.

Make sure all tires are properly inflated. Some systems have a reset button inside the glove box that needs to be pressed and held down for three seconds with the ignition on. If your 2008 Ford Explorer uses an indirect system, the system must be reset using a magnet or scanning tool by either a dealer or one you have bought yourself. If your 2008 Ford Explorer uses a direct system, resetting the tire pressure sensors may be as simple as pushing a reset button on the dash or following a menu as indicated in your owner's manual. Though they do not affect tire balance nor corrode, they must be reset with a scanning tool whenever tires are replaced or rotated. They monitor the speed of the rotations of the wheels for any change since underinflated tires are smaller and rotate faster. The sensors are located within the ABS braking system and/or the speed sensors of each wheel. The indirect TPMS is used more often in Europe. However, these devices can throw off the balance of a tire and can corrode in severe weather conditions. These devices transmit the information to an on-board computer. The sensors are located in the tire pressure valve, which acts as an antenna, or are wrapped around the rim of the wheel with a band clamp. The direct TPMS is used more often in the United States.

Xplorer 2008