Wireless devices hidden in the tires?Hidden wires somewhere?Do they have batteries that run out or do they somehow get power through the rotors?
RFID type chip transmitter devices in the valve stems with an RFID reader mounted to the vehicle by the tire. The BCM usually monitors each tire every 4 revolutions so it may take 4 revolutions for a low pressure flag to clear after you air up the tire.
TPMS (Tire Pressue Monitoring Sensors) are inside the tires and are modules that are part of the valve stem. At least in my tires they are. Yes, they have batteries. Most tire places charge a small extra fee to rebuild them when replacing the tires.
