The tire pressure warning light is a little yellow icon that is roughly a cut away view of a tire with an exclamation point in the middle. If a tire’s pressure falls 25% below 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 computer controlled tire warning system, also known as the Tire Pressure Monitoring System (TPMS), can use several different methods to track the tire pressure. Some use the anti-lock brake sensors i.e. 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, i.e. a low tire will have a smaller diameter than a properly inflated tire.  Other systems use pressure sensors inside the tire itself that can send a signal that is received by a transducer that alerts and triggers the tire warning system and illuminates the light when the tire pressure falls below a specified point.

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 1-2 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

It is highly recommended to read the portion of your vehicle owner's manual that covers the Tire Pressure Monitoring System. This way if, or when, the tire warning light comes on, you'll know exactly what needs to be done for your vehicle. 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 and alert the driver when the tire pressure drops to 0 PSI (pounds per square inch).

Depending on your tire warning system, "Tire low" may be displayed 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 highway and find a safe place to stop. If the vehicle is being driven on a roadway with slower speeds, such as a city street, then pull the vehicle over to a safe location as well. If it is possible to do in a safe manner, get out and observe the condition of all four tires. If any of the tires show any signs of 'sagging' due to improper tire pressure, then call for service because driving an under inflated tire will destroy the sidewalls which will then make the tire unsafe. A tire with a damaged sidewall can fail without any warning and be catastrophic at any speed above typical city speeds of 25 mph.

Sometimes it is difficult or impossible to tell if a tire is low simply by looking, and 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 any 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 worked on by a repair shop, such as brake system work, a tire rotation, a CV Boot/Axle Replacement or even an oil and filter change then most likely 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, then when it is safe to do so, 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 – made a requirement that by 2008 all four wheeled vehicles weighing 10,000 lbs or less must be equipped with a tire pressure warning system. This system alerts the driver if the tire pressure in any of the four tires drops 25% or more.

The tire pressure warning system was required by Congress 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 are 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. This uncontrollable condition often results in a vehicle roll over situation that involves other innocent 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 operator had been warned that one or more of their tires were losing pressure, they could have safely driven their vehicle to a safe location where they could have fixed the tire pressure problem.

Studies in Europe and other countries have shown that 75% of vehicle operators have no idea about the condition of their tires in terms of tread life and also whether or not there is the correct tire pressure in all four tires.

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