A tire pressure monitoring system (TPMS) is an electronic system in a vehicle that notes the car's tire air pressure, and alerts the driver if the tire pressure in one or more tires falls below the recommended pressure. There are two different types of TPMS: direct and indirect.
A direct TPMS relies on a sensor located in the wheel to measure the tire's air pressure, while an indirect system works in conjunction with the vehicle's antilock braking system wheel speed sensors. With a direct TPMS, the sensor alerts the driver when the tire's air pressure falls to 25 percent below the manufacturer's suggested level. The falling air pressure alerts the sensor, which then transmits the information to the vehicle's computer system. Once the information reaches the car's computer system, it often automatically triggers the car's dashboard indicator light. With an indirect TPMS, any tire with low wheel pressure rolls at a different speed than the tires with a normal amount of air pressure. Once this fault is detected by the computer system in the vehicle, the dashboard indicator light usually turns on.
Both types of TPMS systems are available with the factory model of the car or as an aftermarket addition to the vehicle. Tires with low air pressure can lead to traffic accidents or a decrease in fuel economy if not detected and corrected in a timely manner.