Big changes between high and low temperatures that happen in the fall can trigger an alert on your dashboard—one that may leave some drivers wondering what to do next.

If you see a cutaway tire with an exclamation point, it means your tire pressure monitoring system has gone off. Falling temperatures can trigger the system because tire pressure declines with the thermometer.

“In round numbers, tire pressure changes +/-1 pound per square inch for every corresponding change of +/-10 degrees Fahrenheit,” says Woody Rogers, director of tire information at retailer Tire Rack. “As an example, if you set your pressure today at 60 degrees, and tomorrow morning it is 30 degrees, your tire pressure will be about 3 psi lower.”

But not all systems work the same way or provide the same amount of information. Here’s what you need to know if yours goes off.

What do I do when that warning light comes on? 
Pull over as soon as you can and check your tire pressure (some systems give a generic warning, while others give you specifics on pressure). Driving on improperly inflated tires can affect everything from fuel mileage—underinflated tires are less efficient—to how your car rides and steers, particularly in an emergency situation.

And remember that the warning light can come on and then go off.

“Just because the light went out doesn’t mean your tires are properly inflated,” Rogers says.

Of course, regular tire pressure checks always are a good idea, regardless of what your tire pressure monitoring system says.

“You still need to check your tire pressure monthly,” says Gene Petersen, who runs tire testing at Consumer Reports.

More on Tires

What is a tire pressure monitoring system? 
The tire pressure monitoring system does exactly what you’d think—it keeps track of the air pressure in each of your car’s tires. The systems work independently in each wheel, and set off a warning light (the tire/exclamation point icon) on the instrument cluster if the pressure is too low.

The typical trigger happens when the pressure is 25 percent lower than the manufacturer’s recommended cold tire pressure, Petersen says, which can be measured when the tires have been sitting for at least three hours. You can find the recommended pressure level by checking your owner's manual or by reading the sticker on the driver's door jamb.

There are two main flavors of tire monitoring systems: direct systems and indirect systems.

“The vast majority of TPM systems in the U.S. are the direct type, which has a pressure sensor/transmitter mounted to the wheel,” says Rogers. “The sensor/transmitter is most often attached to the end of the valve stem on the end that is out of sight, inside the tire’s air chamber. A small number of direct systems have the sensor/transmitter separate from the valve stem.”

These are attached to the inside of the wheel with a metal strap.

Indirect systems get their information by comparing how the vehicle’s tires are turning in tandem, Rogers says.

“These indirect systems use the wheel speed sensors [part of the antilock brake system] to monitor the revolutions of all four tires as the vehicle moves,” he explains. “The central processor of the indirect system uses advanced algorithms to sense small changes in tire rotation speed created by different inflation pressures among the four wheel positions.”

tpms tire pressure monitoring system
The tire pressure monitor can alert when tire pressure is significantly below the correct level.

If I get new tires do I need new sensors? 
Not necessarily. Tire pressure sensors aren’t permanently affixed to the old tires and they can be reused if they’re still working well.

But, says CR’s Petersen, “typically the battery will last about 10 years and then the sensor needs to be replaced.”

Are TPMS sensors expensive?
The cost depends in part on the work you’re having done, says Tire Rack’s Rogers. If you’re already getting new tires, the sensors will be within easy access and take only a short amount of time and labor to change.

It “becomes a lot more expensive to replace when you aren’t having a tire change done,” says Rogers.

The average price for a direct sensor, according to Rogers, can range from $40 to $70 per sensor, for most common vehicles.

Rogers says that some “niche vehicles” might have sensors that cost hundreds of dollars, but these “very specialized sensors” are extremely uncommon.

checking tire pressure
Even with tire pressure monitoring systems, it is important to manually check pressures monthly.