What is TPMS
As its name suggests, a tire pressure monitoring system is more than a single part. In fact, TPMS involves a valve and a sensor, and it's also important to know that not all TPMS systems are created equal. There are two kinds of TPMS technology–indirect and direct. Indirect TPMS approximates tire pressure indirectly by using data from the vehicle's antilock brake system (ABS).
Direct TPMS is a warning system that warns a vehicle's operator of an unsafe change in the air pressure in one or more of the tires. Readings are provided by pressure sensing transmitters mounted inside each tire and sent to a central computer (ECU) for display on the dashboard. A warning indicator light on the instrument panel and an audible warning notify the driver if a 20%-25% (depending on regional legislation) drop in pressure occurs.
How a Tire Pressure Monitoring System Works
In either case, if a tire is detected to be underinflated by 20%-25% (depending on regional legislation) or more, an alert lights up on your dashboard. But with direct TPMS, drivers are alerted sooner and–if the car is equipped with the four-tire TPMS display–can even see readings for each tire. One of the largest downsides of an indirect TPMS system is that it cannot detect when all four tires are low in pressure, which can happen quite frequently if tire pressure is not checked on a regular basis.