TPMS Explained: How Tire Pressure Monitoring Systems Work

What a TPMS does

A tire pressure monitoring system is designed to warn drivers when one or more tires is significantly under-inflated. Federal Motor Vehicle Safety Standard No. 138, found at 49 CFR 571.138, defines a TPMS as a system that detects when a vehicle's tires are significantly under-inflated and illuminates a low tire pressure warning telltale. The standard applies to passenger cars, multipurpose passenger vehicles, trucks, and buses with a gross vehicle weight rating of 10,000 pounds or less, excluding vehicles with dual wheels on an axle.

The regulatory requirement is specific: the system must illuminate the warning light no more than 20 minutes after the inflation pressure in one or more tires drops to 25 percent below the vehicle manufacturer's recommended cold inflation pressure, or to the pressure specified in a table in the standard, whichever is higher. The light must stay on as long as the pressure remains low and the ignition is in the On or Run position.

The purpose of this requirement is straightforward. Under-inflated tires handle poorly, wear unevenly, and can overheat at highway speeds. A TPMS gives you an early warning so you can address the problem before it becomes dangerous. If you are shopping for replacement tires, knowing how your TPMS works helps you choose the right approach to maintenance. See our guide to the best car and SUV tires for options that suit your vehicle.

Direct versus indirect TPMS

There are two main ways vehicle manufacturers implement tire pressure monitoring: direct and indirect systems. Each has different hardware, different strengths, and different maintenance needs.

A direct TPMS uses a small pressure sensor mounted inside each wheel, typically attached to the valve stem. The sensor measures the actual air pressure in the tire and transmits that data wirelessly to a receiver in the vehicle. The display may show each tire's pressure numerically, or it may simply trigger a warning when one falls below the threshold. Direct systems are generally more accurate and can identify exactly which tire is low, but the sensors contain batteries that eventually wear out and require replacement.

An indirect TPMS does not measure pressure directly. Instead, it uses the anti-lock braking system's wheel speed sensors to compare the rotational speed of each wheel. A tire that is under-inflated has a slightly smaller rolling radius and spins faster than a properly inflated one. The system detects this difference and triggers the warning light. Indirect systems are less expensive to produce and have no in-wheel components to service, but they cannot show exact pressure readings and may need to be reset after tire rotation, replacement, or pressure adjustment.

The Federal Motor Vehicle Safety Standard No. 138 does not mandate which technology a manufacturer must use. It only requires that the system meet the detection and warning timing requirements. Both types can comply. The distinction matters most when you are buying tires or having service work done: direct sensors need to be handled carefully during mounting and dismounting, while indirect systems may need a reset procedure after service.

Reading the TPMS warning light

Standard No. 138 requires that the low tire pressure warning telltale be mounted inside the occupant compartment, in front of and in clear view of the driver. It must be identified by one of the symbols shown for the Low Tire Pressure Telltale in Standard No. 101. The telltale must illuminate as a check of lamp function when the ignition is turned to the On or Run position with the engine off, so you can verify the light bulb or LED still works.

Some systems use a single warning light for all four tires. Others use a telltale that identifies which specific tire is under-inflated. The standard says that in the case of a telltale that identifies individual tires, each tire in the symbol must illuminate when the tire it represents is under-inflated to the required threshold.

There is also a separate warning for system malfunctions. The standard requires the vehicle to have a telltale that warns the driver no more than 20 minutes after a malfunction that affects the generation or transmission of control or response signals in the TPMS. This can be a dedicated telltale labeled TPMS, or a combined low tire pressure and malfunction telltale. If the light comes on and stays on at startup, or if it blinks, the system itself may need service even if the tires look fine.

It is important to distinguish between the TPMS light and other dashboard warnings. A TPMS warning relates specifically to tire pressure or to the monitoring system itself. If the light stays on after you have inflated the tires to the recommended pressure, the system may need to be reset or one of the sensors may have failed.

Why tire pressure matters

The TPMS is a warning system, not a substitute for manual pressure checks. Keeping tires inflated to the vehicle manufacturer's recommended cold inflation pressure is one of the most effective ways to maintain safe handling and maximize tire life. According to the U.S. Department of Energy's fueleconomy.gov, you can improve your gas mileage by 0.6 percent on average, and up to 3 percent in some cases, by keeping tires inflated to the proper pressure. Under-inflated tires can lower gas mileage by about 0.2 percent for every 1 psi drop in the average pressure of all tires.

The correct pressure for your vehicle is not the maximum pressure printed on the tire sidewall. It is the vehicle manufacturer's recommendation, usually found on a sticker in the driver's side door jamb or in the glove box, as well as in the owner's manual. Federal Motor Vehicle Safety Standard No. 110, found at 49 CFR 571.110, references the vehicle placard and tire inflation pressure label as the source of the manufacturer's recommended cold tire inflation pressure. The standard also requires that the vehicle normal load on the tire not exceed 94 percent of the load rating at the recommended cold inflation pressure. Choosing the right tire for your vehicle also depends on proper inflation; see our picks for the best car and SUV tires for guidance.

The relation between tire pressure and load is one reason the TPMS threshold is tied to the manufacturer's recommendation rather than a fixed pressure. A tire that is slightly low for light loads may be significantly under-inflated for heavy loads. The TPMS is designed to catch the significant under-inflation that can lead to tire failure, but it does not relieve you of the responsibility to check pressure regularly, especially before long trips or when carrying heavy cargo.

If a TPMS warning light comes on, check the tire pressure with a reliable gauge as soon as possible, and inflate the tires to the recommended pressure while they are cold. If the light stays on, have the system inspected by a professional.

Maintaining your TPMS sensors

TPMS sensors are electronic components with a limited service life. Direct system sensors are battery-powered, and the batteries are not designed to be replaced separately. Most sensors last between five and ten years, though this can vary by manufacturer. When the battery dies, the sensor must be replaced. If the TPMS malfunction light comes on steadily or blinks, a dead sensor is a common cause.

The Tire Industry Association, a trade group that provides technical training for tire service technicians, emphasizes that consumer education about TPMS is part of proper tire maintenance. Their training and consumer safety resources cover tire pressure monitoring system servicing. That is a reminder that TPMS work is a routine part of tire service, and a good tire shop should have the tools and training to handle it.

When you buy new tires, a set of four is a good time to have the TPMS inspected. Many shops will check sensor health as part of the installation process. If a sensor is close to the end of its service life, replacing it while the tire is off the wheel is easier and avoids a second trip to the shop. If you plan to keep the vehicle for several more years, proactive replacement of aging sensors can prevent a malfunction light from appearing shortly after new tires are installed.

If you rotate tires, be aware that a direct TPMS sensor goes with the wheel it is mounted on. If your vehicle has a system that identifies each tire's position, the sensors need to be relearned or reprogrammed after rotation so the display shows the correct position. The tire rotation pattern is another reason to keep records of where each sensor is mounted. Consult tire rotation patterns for guidance on staying organized.

When to replace sensors

There are several signs that a TPMS sensor needs replacement: the malfunction light is on, the system does not display a reading for one or more tires, or the sensor has been in service for a decade or more. Some vehicles will show a tire pressure reading that is obviously wrong, such as a reading well above or below what a verified gauge shows. That is a sign the sensor is drifting or failing.

The Federal Motor Vehicle Safety Standard No. 138 requires that the system detect malfunctions that affect the generation or transmission of control or response signals. This means a dead sensor will typically trigger the malfunction telltale within 20 minutes of the ignition being turned on. You cannot ignore this light and expect the system to protect you. A vehicle with one dead sensor has no warning coverage for that tire, which defeats the purpose of the system.

Replacement sensors are available from tire dealers and auto parts stores. The vehicle's owner manual or a qualified technician can tell you which sensor is right for your vehicle. Some aftermarket sensors are programmable, while others are pre-programmed to match specific vehicle models. The sensor must be installed properly and, on many vehicles, registered with the vehicle's computer using a TPMS scan tool. This is a job for a professional, since the sensor must be torqued correctly and the system must be reset to clear the malfunction light.

When you replace tires, ask the shop about the condition of the TPMS sensors, especially if your vehicle is older. It is also worth asking about the tire's compatibility with your TPMS. Some tires are designed to minimize pressure loss, but all pneumatic tires lose air slowly over time. See tire pressure basics for how often to check and what inflation pressure to use.

What to pick for your use

Questions

Why did my TPMS light come on when the tire looked fine?

A tire can be significantly under-inflated without looking flat. The TPMS threshold is 25 percent below the recommended cold inflation pressure, which is a large drop that is not always easy to spot visually. Check the pressure with a gauge and inflate to the recommended pressure if it is low. See tire pressure basics for more on checking.

Can I drive with the TPMS light on?

You can drive a short distance to a safe location or a service station, but you should not ignore the light. Under-inflated tires handle poorly and can overheat. Inflate the tires to the recommended cold pressure as soon as possible and have the system checked if the light stays on.

Do I need special tires for TPMS?

No. TPMS works with standard pneumatic tires. The sensors are mounted on the wheel, not built into the tire. However, some tire repair methods and mounting procedures must account for the sensor to avoid damaging it. Let the shop know your vehicle has TPMS. You can find suitable tires in our guide to the best all-season tires.

How often should I check my tire pressure?

Check tire pressure at least once a month, and before long trips. Use a reliable gauge and check when the tires are cold, meaning the vehicle has not been driven for at least three hours. The recommended cold inflation pressure is on the vehicle placard in the driver's door jamb.

Will a shop know how to service TPMS?

Most tire dealers and repair shops have TPMS tools and trained technicians. The Tire Industry Association offers training and certification programs that cover TPMS servicing. Ask the shop about their TPMS experience when you schedule tire service.