Tire Pressure Monitoring Systems (TPMS)

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A tire pressure monitoring system, or "TPMS" for short, describes a series of sensors placed on each wheel — underneath the tire — and used to continuously monitor the air pressure in a given tire. If the air pressure falls dangerously low (more on that below), the sensor will notify the driver with a TPMS warning light on the instrument panel.

TPMS types
A brief TPMS sensor history
What is the TPMS light?
Recognize the TPMS light. Don't ignore it
What to do when the light comes on
The dangers of an underinflated tire
Video: How to manually check your tire pressure

TPMS types

There are two types of tire pressure monitoring systems: indirect and direct.

Indirect TPMS: The lower-cost indirect TPMS doesn't use sensors in the tires. Rather than monitor air pressure, it uses the antilock braking system's wheel-speed sensor to detect that one tire is rotating faster than the others. An underinflated tire will have a smaller circumference, so it has to roll faster to keep up, which triggers the system to notify the driver.

Indirect tire pressure monitoring systems do have some notable drawbacks. They have a greater margin of error, especially when using different-sized tires. They also don't show you which tire is low on pressure or warn you if all four tires are losing pressure at the same rate — something that can happen in colder climates.

When the TPMS light is illuminated on a vehicle with an indirect monitoring system, you must check each tire's pressure with a tire gauge to determine which one is leaking or needs air.

Direct TPMS: A direct TPMS setup uses sensors placed inside each tire to measure their individual pressure. Such systems are usually accurate to within 1 psi. Current direct systems use a sensor mounted to the wheel or tire valve, which 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.

• High-line systems have sensors mounted in each of the wheel wells. The instrument cluster can then display each tire's individual tire pressure.

• Low-line systems are found on less expensive cars and will only prompt the low-pressure warning light. Similar to an indirect system, it is then up to the driver to figure out which tire is low.

How can you tell whether your car — or a used car you're considering 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 tend to have direct systems. If the idea of a high-line system appeals to you, ask the salesperson to help you check the instrument cluster screen during your next test drive. Go through the menus, and if you see the individual tire pressure readouts, you're all set. If not, know you'll need to invest in a good tire pressure gauge to handle the task yourself. They aren't expensive, and one should be in your glovebox.

A brief TPMS history

The technology dates back to the mid-1980s in Europe, and it made its way to the U.S. a few years later as an optional feature on the 1989 Chevrolet Corvette. TPMS sensors eventually became standard on all 2008 and newer vehicles thanks to the federal TREAD Act, which Congress enacted in 2000 after rollover incidents involving the Ford Explorer and Firestone tires. Some 2006 and 2007 model-year vehicles might have TPMS, but this can vary based on the installed features.

What is the TPMS light?

The TPMS light can look like either a cutaway icon of a tire with an exclamation point or an overhead view of a car with the affected tire highlighted. The warning light's meaning depends on which type of TPMS the vehicle is equipped with.

With a direct tire pressure monitoring system, the TPMS light will warn the driver when the air pressure in any tire drops at least 25% below the recommended cold tire inflation pressure, noted in the placard on the driver's side doorjamb. Once the affected tire is inflated, the TPMS light should turn off on its own after a few miles of driving. If it doesn't, the tire may have an air leak.

Vehicles with an indirect tire pressure monitoring system will warn the driver when a single tire has lost at least 25% of its inflation pressure relative to the other three tires. Once the tires are properly inflated, there should be a button in the car that resets the sensors so the TPMS light can turn off.

In either scenario, this is well below the pressure required for safe driving, according to the American Automobile Association (AAA). The association urges drivers to keep their tires inflated to the vehicle’s posted pressure rather than relying solely on the TPMS light to determine when a tire needs to be reinflated. It is not a substitute for regularly checking your tire pressure.

Recognize the TPMS light. Don't ignore it

In theory, these tire pressure monitoring systems are features that help drivers maintain the safety of their cars. But they're only effective if drivers can identify the light and are still vigilant about checking their tire pressures. A 2018 survey of over 1,000 U.S. drivers commissioned by Goodyear Auto Service and Just Tires showed that 49% of younger drivers and 39% of drivers overall could not recognize the TPMS warning symbol.

An older study by Schrader International, a company that manufactures TPMS systems, found that roughly 42% of drivers polled admitted to rarely checking the tire pressure.

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

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, the monitoring system could be malfunctioning. 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. According to Tire Rack, a 10-degree drop in ambient temperature results in the loss of about 1 psi. 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. There's a chance your tires 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.

The dangers of an underinflated tire

A tire that's underinflated by just 5 psi can potentially fail. An underinflated tire flexes more than a properly inflated tire, creating heat. Excessive heat can break down components and chemical bonds inside a tire. It's similar to 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.

Besides 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.

How to manually check your tire pressure

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 for you.

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 trunk lid, fuel door, glovebox, center console lid, passenger's doorjamb, or in your owner's manual. It won't be written on the tire itself.