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If you own a car built after 2007, you have a tire pressure monitor system at work. You know it best as the horseshoe-shaped light (it's actually a stylized, flattened tread and sidewall of a tire) with an exclamation point in the center. It's also the light that can plague you by coming on and going off at inexplicable times, and sometimes signaling an expensive trip to the dealership.
But this indicator light is the visible part of a larger pressure monitoring system, and it's there to help. When it's on, it's telling you that your tires need air.
The various tire pressure monitoring systems (or TPMS) used by auto makers are designed to monitor the air pressure in a car's tires. The idea behind a TPMS is primarily safety-related -- underinflated tires offer a less stable ride, and they're more prone to possible blowouts. By calling attention to an "underinflation event," the system can prompt the driver to inflate the tire (or tires) to the proper levels.
Sensors within the tire, or on the car, send information to one or several modules in the car. These modules are programmed with a range of acceptable circumstances. For direct tire pressure monitoring, this is often between 28 and 35 pounds per square inch (psi) of air in the tire.
This rather innocent light has a tragic origin. During the late 1990s, more than 100 automotive fatalities were attributed to Firestone tires that lost their tread when they were run underinflated, and friction heated them beyond their capability to handle. The tires blew out or delaminated, and this led to the rollover of the vehicles they were on. Most of those vehicles were Ford Explorers, and many times one or more of the occupants died.
The fatalities led to two major changes to the automotive industry. The first was the Transportation Recall Enhancement, Accountability and Documentation Act (The TREAD Act). The act, later signed into law, required tracking of, and response to, any possible danger signs from vehicles that would require a recall or posed a safety risk.
The second major addition was the requirement of a TPMS system on all cars built after 2007 in the United States. Like most quickly-introduced changes, there were problems with the systems. But as technology improves, and engineers refine how the systems function, they're becoming smoother and more reliable.
Read on and find out how the systems actually see inside your tire, what they see, and how your car reacts to the information.