<P> After the TREAD Act was passed, many companies responded to the market opportunity by releasing TPMS products using battery - powered radio transmitter wheel modules . </P> <P> The introduction of run - flat tires and emergency spare tires by several tire and vehicle manufacturers has motivated to make at least some basic TPMS mandatory when using run - flat tires . With run - flat tires, the driver will most likely not notice that a tire is running flat, hence the so - called "run - flat warning systems" were introduced . These are most often first generation, purely roll - radius based iTPMS, which ensure that run - flat tires are not used beyond their limitations, usually 80 km / h (49.7 mph) and 80 km (49.7 miles) driving distance . The iTPMS market has progressed as well . Indirect TPMS are able to detect under - inflation through combined use of roll radius and spectrum analysis and hence four - wheel monitoring has become feasible . With this breakthrough, meeting the legal requirements is possible also with iTPMS . </P> <P> Indirect TPMS do not use physical pressure sensors but measure air pressures by monitoring individual wheel rotational speeds and other signals available outside of the tire itself . First generation iTPMS systems are based on the principle that under - inflated tires have a slightly smaller diameter (and hence higher angular velocity) than a correctly inflated one . These differences are measurable through the wheel speed sensors of ABS / ESC systems . Second generation iTPMS can also detect simultaneous under - inflation in up to all four tires using spectrum analysis of individual wheels, which can be realized in software using advanced signal processing techniques . The spectrum analysis is based on the principle that certain eigenforms and frequencies of the tire / wheel assembly are highly sensitive to the inflation pressure . These oscillations can hence be monitored through advanced signal processing of the wheel speed signals . Current iTPMS consist of software modules being integrated into the ABS / ESC units . </P> <P> iTPMS cannot measure or display absolute pressure values, they are relative by nature and have to be reset by the driver once the tires are checked and all pressures adjusted correctly . The reset is normally done either by a physical button or in a menu of the on - board computer . iTPMS are, compared to dTPMS, more sensitive to the influences of different tires and external influences like road surfaces and driving speed or style . The reset procedure, followed by an automatic learning phase of typically 20 to 60 minutes of driving under which the iTPMS learns and stores the reference parameters before it becomes fully active, cancels out many, but not all of these . As iTPMS do not involve any additional hardware, spare parts, electronic or toxic waste as well as service whatsoever (beyond the regular reset), they are regarded as easy to handle and very customer friendly . </P>

When do tire pressure sensors need to be replaced