There are three main types of tire pressure monitoring systems offered today:
Direct TPMS:Direct systems monitor the pressure and temperature from within the tire using actual sensors to communicate with the vehicles on-board receiver.
Indirect TPMS:Indirect systems does not use a physical pressure sensor within the wheel to monitor the individual pressure, but in fact uses the vehicles ABS system to measure the rotational speeds of each tire. Minor tire pressure differences can cause one or more of the tires to rotate at a different rate than the others which is detected by the system and will cause the TPMS light to turn on. Defective wheel speed sensors or driving in slippery conditions can also cause the TPMS light to turn on with this type of system.
Hybrid TPMS:Internal tire sensors work in conjunction with wheel speed sensors to help tell the driver (via a dashboard indicator) which tire is underinflated.