What You Need to Know About Tire-Pressure Monitoring Systems

If you're buying or driving a 2008 or newer car, truck or SUV, it has a tire-pressure monitoring system (TPMS), which uses sensors to continuously monitor pressure in the tires and warn you with a dashboard symbol when tire pressure is dangerously low. The feature is standard on all 2008 and newer models, thanks to the TREAD Act, which Congress enacted in 2000 after rollover incidents involving the Ford Explorer and Firestone tires. Some 2006 and 2007 model-year vehicles also have TPMS.

The TPMS symbol is either a cross-section of a tire with an exclamation point in it or an overhead view of a car with all four tires exposed. Because of a variety of considerations from tire companies and automakers, a TPMS warning light isn't required to come on until a tire is 25 percent below the manufacturer's recommended tire pressure. That's also well below the pressure required for safe driving, according to the American Automobile Association. So here's the short version: Do not use the TPMS warning light as a substitute for regularly checking your tire pressure and keeping it at the recommended level. That's not its purpose.

Recognize the Light. Don't Ignore It

In theory, a TPMS is a feature that helps drivers understand the safety of their cars. But it's only effective if drivers can identify the light and are still vigilant about checking their tire pressures. A 2014 study by Schrader International, a company that manufactures TPMS systems, found that 42 percent of drivers are unable to identify the low tire-pressure warning light on the instrument cluster. Roughly the same percentage of those polled admitted to rarely checking the tire pressure.

For those who do recognize the TPMS light, 21 percent said that when they stop to check the low tire, they only give it a visual inspection rather than using a tire pressure gauge, the study said. Worse yet, roughly 10 percent admitted to ignoring the light altogether.

People who rely on the TPMS to warn them about low pressure are taking chances. A tire that's underinflated by just 5 psi can potentially fail. An underinflated tire flexes more than a properly inflated tire, and that creates heat. Excessive heat can break down components and chemical bonds inside a tire. It's much like bending a wire coat hanger: Bend it far enough and long enough, and it will heat up and snap. It's especially important to be vigilant about tire pressure when the weather is hot and vehicle speeds are high.

In addition to being a safety hazard, low tire pressure decreases fuel economy and causes tires to wear out more quickly. These are all reasons to be vigilant about checking tire pressure at least monthly and to not rely on a TPMS to do the job.

What to Do When the Light Comes On

When the TPMS light illuminates, you should check the pressure on all tires. Only one may need air, but checking all of them is a good habit. If the light is flashing, there could be a malfunction with the TPMS system. It could also mean that you are using a spare tire and the vehicle cannot detect the sensor of the original wheel.

Temperature also can affect tire pressure and trigger your TPMS light. When the weather is cold, the tire pressures drop. A 10-degree drop in ambient temperature results in the loss of about 1 psi, according to Tire Rack. When you first start up the car after a cold night, you might see the TPMS symbol illuminate for a short time and then shut off. Your tires probably had marginal low pressure to begin with, dipped below the warning-light threshold overnight and then rose as the tires heated up, according to the National Highway Traffic Safety Administration's tire safety page. In any case, check your tire pressure and add air if necessary.

TPMS Types

There are two types of TPMS: indirect and direct. The lower-cost, indirect TPMS doesn't actually monitor air pressure. Rather, the indirect TPMS uses the antilock braking system's wheel-speed sensor to detect that one tire is rotating faster than its mates. (An underinflated tire has a smaller circumference so it has to roll faster to keep up.) Thus, there's a large margin of error in indirect systems.

Meanwhile, a direct TPMS measures a tire's actual pressure. Such systems are usually accurate to within 1 psi. Current direct systems use a gauge mounted to the wheel or tire valve. This gauge sends a signal to the car's computer. How the data is interpreted and displayed in the car with a direct TPMS depends on whether it is a low-line or high-line unit.

A high-line system has sensors mounted in each of the wheelwells and displays the individual pressure of each tire on the instrument cluster. Low-line systems are found on less expensive cars and will only prompt the low-pressure warning light. It is then up to the driver to figure out which tire is low. Much more useful to drivers, clearly, are systems that actually display the pressure of each tire.

If that's what you want, make sure you look for a high-line system when shopping for a new car. Take a moment to check the instrument cluster screen on your next test drive. Go through the menus and if you see the individual tire-pressure readouts, you're all set. If not, just know that you'll need to invest in a good tire pressure gauge so that you can handle the task yourself. They aren't expensive and one should be in your glovebox.

How can you tell whether your car — or a used car you're thinking about buying — has an indirect or direct TPMS system? There's no easy way to tell without looking inside the tire, but as a general rule, vehicles from the 2008 model year or newer have direct systems.

Be Your Own Monitor

To sum up: The tire-pressure monitoring system is only there to warn you of a puncture or an active air leak. It's your responsibility as a driver to check your tire pressures monthly or have someone else check them.

On new cars, the automaker's recommended pressure is on a placard on the driver's doorjamb. On older cars it can be on the trunklid, fuel door, glovebox, center console lid, passenger's doorjamb or in your owner's manual. It is not on the tire.