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You're driving along and you notice a new light on your dashboard display that you haven't seen before. It looks like a horseshoe. You think to yourself "I'm driving a car, not a horse, what's happening?" Sometimes this light may be accompanied by a alarm sound that's not easy to miss.
What's happening is your car is trying to tell you that something is wrong with your tires/wheels. Your car has four tire pressure sensors (1 per tire) and when they detect that your tire is getting low on air, the sensor will come on, alerting you.
There are several reasons why the tire pressure sensor may come on, including wheel damage or a flat tire, but it's important that you don't just ignore the light as you can do significant damage to the tire, rim and worse by driving on a deflated tire.
Pull over at a gas station or any place that has a cheap compressed air machine. Unscrew the air chuck and use the machine to fill the tire to the recommended PSI which is listed in your vehicle owner's manual as well as the inside of the driver's side door jamb. Once you've put enough air in your tire, the little horseshoe should disappear and you can go back to living a carefree lifestyle.
If there are more serious issues with your wheel and tire, you may want to have your car towed to the nearest auto shop. Don't risk driving on a damaged wheel or tire. Your tires are literally the only connection between your car and the road and not something to be taken lightly.
One last thing to note; if you put different wheels on your car for snow season, this can also cause the light to come on. Check your vehicle owner's manual or with your vehicle manufacturer to ensure that this is not unusual to the vehicle as you don't want to ignore a "true" alert.