That little horseshoe-shaped icon with an exclamation point in the middle lighting up on your dashboard is the Low Tire Pressure Symbol. It’s your car’s way of telling you that one or more of your tires isn’t inflated to the proper pressure. This system, known as the Tire Pressure Monitoring System (TPMS), is a crucial safety feature in modern vehicles. It constantly monitors the air pressure in your tires and alerts you when it drops too low.
Typically, the low tire pressure symbol illuminates when a tire’s pressure falls 25 percent or more below the manufacturer’s recommended cold-tire pressure. This “cold-tire pressure” is measured when your tires have been stationary for at least three hours, ensuring an accurate reading unaffected by heat buildup from driving. You can easily find the correct tire pressure for your vehicle on a sticker usually located on the driver’s side doorjamb or in your owner’s manual. Maintaining proper tire pressure is not just about safety; it also impacts fuel efficiency and tire wear.
There are two main types of Tire Pressure Monitoring Systems in use today: direct and indirect systems.
Direct TPMS
The most common type, especially in the U.S., is the direct TPMS. These systems use physical pressure sensors installed inside each tire. Most often, the sensor and transmitter are integrated with the valve stem, positioned inside the tire where they’re protected. Some direct systems might have the sensor attached to the wheel itself with a band, separate from the valve stem. Direct TPMS provides real-time, accurate pressure readings directly from each tire.
Indirect TPMS
Indirect TPMS, on the other hand, doesn’t use physical pressure sensors. Instead, it relies on the vehicle’s anti-lock braking system (ABS) wheel speed sensors. These systems work by monitoring and comparing the rotational speed of each tire. A tire with lower pressure will have a slightly smaller diameter, causing it to rotate at a different speed than the properly inflated tires. The car’s computer uses sophisticated algorithms to detect these subtle differences in wheel speed and infer a drop in tire pressure. While less direct, these systems still effectively alert drivers to potential tire pressure issues.
