A car’s “check engine” light is so mysterious. “Something is wrong!” it visually screams. What is wrong? Is my engine about to explode? Or did I just forget to tighten the gas cap again? The light just sits there, shining at you. “Check it!” Fortunately, there’s an easy way to find out what it’s trying to communicate—for free.

Since 1996, new cars sold in the U.S. are required to have onboard diagnostics (OBD) systems that allow mechanic can plug in a device and receive a code from the car’s computer that narrows down the source of a problem. If you did bring your car straight to the garage or dealership to get checked out, this is likely the first thing they would do.

The device—called an OBD2 or OBD-II scanner—will give them a number they can then look up and find out what might be wrong. This isn’t just for the check engine light, either; the OBD system can provide a wealth of information about what is going on inside your car. You can buy a cheap OBD2 reader to check basic codes yourself, or a fancy one to get more information.

But you don’t need to buy a scanner or book an appointment at the dealership just to find out a code reading. Many auto parts stores will do the job for you for free.

How to get your OBD2 codes read at an auto parts store

Many auto parts stores offer code readings as a free service. Advance Auto Parts, AutoZone, and O’Reilly are among the stores that will. (So do many other chains and independent stores. Check with your local store to find out whether they offer this service, and whether it’s free.)

Knowing the code may not tell you exactly what’s wrong with your car, but it can narrow down the type of problem you’re having. You can see some of the codes here, if you’re curious. For example, there is a code that indicates a leak in the emissions control system. It’s often triggered by your gas cap being loose. But it could also mean that the cap is cracked or that there is a leak somewhere else in the system. There are other codes that would indicate if something is wrong with the catalytic converter, or if the engine is misfiring.

At the store, the person helping you will tell you what the code means, and suggest what to do to fix it. They offer this service because people will often check their diagnostics, then buy a part to fix whatever is wrong. So if the problem is straightforward, you might be on your way pretty quickly—but if not, you’ll at least have a better idea of whether you really need to book that mechanic’s appointment.