Car encyclopedia

Tire Pressure Warning Light

The tire pressure warning light is a little yellow icon that looks like a cut-away view of a tire with an exclamation point in the middle. If a tire’s pressure falls below 25 percent of the pressure specified by the vehicle manufacturer, the tire warning light will illuminate. Some systems will illuminate the tire warning light if the tire pressure is too high.

The Bulb Check: Tire Pressure Warning Light

When the ignition key is cycled from Off-Run-Start and is released back to the Run position after the vehicle starts, the tire pressure warning light should illuminate for 1 to 2 seconds and then go out. If it stays on, there is a problem with the tire pressure in one or more tires—or there is a fault in the tire pressure warning system.

What to Do: Tire Pressure Warning Light

The computer controlled tire warning system—also known as the tire pressure monitoring system (TPMS)—uses several different methods to track the tire pressure. Some use the anti-lock brake sensors (a deflating tire will turn faster than a properly inflated tire and therefore trigger the light). Some systems use sensors in the wheel wells that carefully track the diameter of the tire (a low tire will have a smaller diameter than a properly inflated tire). Other systems use pressure sensors inside the tire itself. When the tire pressure falls below a certain point, the sensor will send a signal to a transducer, which alerts the tire warning system and illuminates the light.

It is important to read the portion of your vehicle owners manual that covers the tire pressure monitoring system. That way, if or when the tire warning light comes on, you'll know exactly what to do. Not only do vehicles come equipped with the tire pressure monitor, some models also have a flat tire monitor. Flat tire monitors are more common on vehicles equipped with run-flat tires. They alert the driver when the tire pressure drops to 0 PSI (pounds per square inch).

Depending on your tire warning system, "tire low" or the actual tire pressure may be displayed. It is a good idea to keep a tire pressure gauge in the vehicle's tool kit in case you need to inspect the air pressure in your tires or recheck the pressure after adding air. Tire pressure gauges are inexpensive and available at nearly all stores with an automotive section, auto parts stores, and in auto dealers' parts departments.

If the tire warning light comes on while driving—especially on the highway—then it is best to pull the vehicle off the road and find a safe place to stop. If it is possible to do so in a safe manner, get out and observe the condition of all four tires. If any of the tires show signs of sagging due to improper tire pressure, then call for service—driving an under inflated tire will destroy the sidewalls and make the tire unsafe. A tire with a damaged sidewall can fail without warning and be catastrophic at any speed above 25 MPH.

Sometimes it is difficult or impossible to tell if a tire is low simply by looking at it. This is why a tire pressure gauge is useful. Run-flat tires will not show signs of low pressure and loaded vehicles not equipped with run-flats can often look as though the tires are low. Vehicles parked on a slope can have a tire that appears low on pressure when it might be normal.

If the tire warning light stays on while the vehicle is being started, then shut it off and, if it is safe to do so, walk around the vehicle and observe all four tires. If any of the tires show signs of sagging from improper inflation, then call for service. If you are sure that the tire pressure in all four tires looks completely normal, then most likely the problem is in the tire warning system itself. Have the vehicle diagnosed and inspected at a qualified repair shop.

If the tire warning light comes on soon after the vehicle was serviced at a repair shop—for brake system work, a tire rotation, a CV boot/axle replacement, or even an oil and filter change—then it's likely that something was disturbed that tripped one or more of the tire pressure sensors, which triggered the warning light. Even topping off the tire pressure (say during an oil change) in one or more of the tires can require a re-set procedure that must be performed or the tire warning light system may illuminate the warning light.

In some systems, the tire warning light will steadily flash whenever there's a problem within the warning system itself. If this is the case, pull the vehicle to the side of the road in an area where it is safe to walk around and inspect all four tires. If you are sure that all of the tires look like they're at normal tire pressure, then have the vehicle tire pressure warning system inspected and diagnosed at a qualified repair shop. If you find that one or more of the tires are sagging and look low, or you are not sure, then call for service.

Background Information: Tire Pressure Warning Light

The National Highway Transportation Safety Administration (NHTSA) required that all four-wheeled vehicles weighing 10,000 pounds or less must be equipped with a tire pressure warning system by 2008. This system alerts the driver if the tire pressure in any of the four tires drops 25 percent or more.

Congress mandated the tire pressure warning system when it enacted the Transportation Recall Enhancement, Accountability, and Documentation (TREAD) Act in 2000. Congress took this action because mounting evidence indicated that an increasing number of highway accidents were related to tire problems. The evidence showed that drivers are unaware that one or more of their tires are losing pressure until the tire collapses and the vehicle becomes uncontrollable, which often results in a vehicle rollover situation involving other vehicles on the road, including big rig trucks. Some of the worst multi-vehicle and multi-fatality highway collisions are due to one vehicle experiencing a tire failure that was preceded by a steady loss of tire pressure. In most cases, if the vehicle operators had been warned that one or more of their tires were losing pressure, they could have safely driven their vehicle to a location where they could have fixed the tire pressure problem.

Studies in Europe and other countries have shown that 75 percent of vehicle operators have no idea about the condition of their tires in terms of tread life or if their tires are properly inflated.

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