Every vehicle manufactured since 2008 weighing less than 10,000 pounds is equipped with a system that warns the driver of low air pressure in the tires. This system is commonly referred to as the Tire Pressure Monitoring System, or TPMS. When the system senses that the air pressure in a tire drops below a predetermined amount you will receive a warning indicator.
Depending on the vehicle, the low air pressure warning will be either an illuminated light (called a Malfunction Indicator Lamp or MIL), or a message on the vehicle’s information system alerting you to the low air pressure. Some vehicle have both a MIL and a message. If your vehicle has a MIL, it will either be the words “TPMS” or it will be the cross-section of tire with an exclamation point inside of it.
How does the TPMS determine that the air pressure in your vehicle’s tire is low? There are two different type of systems, direct and indirect.
Direct TPMS use an air pressure sensor mounted inside the tire. Most of these sensors are incorporated into the valve stem, but some are attached to the inside of the wheel using a metal band. These sensors are battery powered and broadcast a signal to the vehicle’s electronic control unit (ECU) that identifies the pressure it senses. If the ECU determines that the pressure being reported by one of the sensors is lower than a predetermined amount then a MIL is illuminated or the vehicle’s information system will display a warning. Side Note: The batteries in these sensors typically last between 6 and 10 years and when they die the entire sensor must be replaced.
Indirect systems use the Anti-Locking Brake System (ABS) or Electronic Stability Control System (ESC) to determine if a tire is low on air pressure. This is possible because as a tire loses air its circumference, or the distance around the outside of the tire, decreases causing it to spin faster than the other tires. The vehicle’s ABS/ESC system can detect this difference in rotational speed and subsequently triggers the low air pressure warning.
When TPMS was first introduced, direct and indirect systems were used about the same amount. Over time, it was determined that direct TPMS was more accurate than indirect systems and indirect systems were phased out. Recently, advances in computer technology and the sophistication of the Electronic Stability Control Systems (ESC) have allowed a resurgance of indirect TPMS.