One redditor thought they found a real bargain when they bought a 2014 car with just 4,000 miles. But they were in for a shock the first time they brought it to a mechanic: the original tires had reached their expiration date. Here’s the true danger of expired tires, and how to tell if yours are reaching the end of their life.

The Reddit forum /r/AskMechanics is a popular place for its 209,000 members to post automotive questions for professional mechanics. One redditor had an interesting issue. They’d invested in a ten-year-old car with just 4,000 miles. Eight months and 2,000 miles later they took it into their mechanic and were told, “even though the tires have plenty of tread, they are expired.”

The driver knew to test tire tread depth with a penny and even visually inspect them for dry rot. So they were shocked by this diagnose of otherwise “healthy” tires. They turned to the internet to ask, “Is there any way to really assess them…?” They were told, “Yes. There is a date code on the sidewall.”

Every tire made since the year 2000 has a uniform date code format. The format is four digits printed on the sidewall. The first two digits are for a week (1-52) and the second two digits are for a year. Taken together, these four digits tell you exactly when the tire was manufactured.

So when is a tire “expired?” Most mechanics won’t mount any tire over 10 years old on your car. But certain tire brands actually recommend you replace your tires after just six years. So the actual “expiration date” of a set of tires will come down to the tire company’s recommendation.

Can you drive a car on expired tires? Technically, it may be possible. But according to Edmunds, multiple deadly accidents have been traced back to expired tires. Even actor Paul Walker’s death may have been in-part due to 9-year-old tires.

The rubber in tires dries out over time. Old tires do not provide as much grip and they do not flex as much as they are engineered to. They can suffer a “blow out” when the sidewall explodes. In extreme cases, the tread section can even detach from the rest of the tire.

If your mechanic tells you your tires have expired, they aren’t conning you for money, they are trying to protect you. If you are buying a used car, its always a good idea to find the four digit code on the sidewall of each tire and make certain they were made recently. In addition, savvy tire buyers also check the date codes on “new” tires from a tire shop to make certain those haven’t been on the shelf for more than a year or two.

Next, find out how many months old a ‘new’ car can be, or see why the tire industry is opposed to true ‘expiration dates’ on sidewalls: