7:40, with "Gma" investigates. An important warning for anyone in the market for a used car. You may have thought that odometer rollbacks were a thing of the past. But a new device is making it even... See More
7:40, with "Gma" investigates. An important warning for anyone in the market for a used car. You may have thought that odometer rollbacks were a thing of the past. But a new device is making it even easier to do. ABC's gio Benitez has the story. Reporter: It's the middle of Atlanta's afternoon rush hour, March 2013. And shakaya was on her way to work, in the 2001 used chevy cavalier she just bought, like this one. I'm driving. I'm singing. Reporter: But on that busy highway, her joyride to work was cut short. My car literally stops. Reporter: Without cash for a tow, she's stuck for five hours. Tried to restart it. Nothing is happening. Finally, the car just, like magic, cranked back up. Reporter: So, she turns to the internet and finds the carfax report. There it is. The car she thought had 129,000 miles had far more street cred. It has almost 300,000 miles on it. And I'm livid. Chekia bought the car after seeing this ad on craigslist, advertised at 129,000 miles. The salesman, this man. He said, you can see yourself. I look on the odometer it read 129,000 miles. Reporter: The bill of sale did note that the odometer Reading was not the actual mileage. Chekia says she never saw that line. As it turns out, carfax says about 1 million cars are on the road right now with odometers that have been swapped out or rolled back. How easy is it to do? With this, it took me seconds. Watch as josh shows me how to change the odometer with a device that has legitimate uses but can be bought by anyone. On this 2012 silverado, we have nearly 19,000 miles. I'm going to say, what? 500 miles? Sure. Let's change it. Success. We've got a car -- wait. Now, it says 499 miles. Yes. Reporter: Even less than I programmed it for. And this car got more expensive. Reporter: Car dealers can easily swap out odometers for newer ones. Just like in chekia's case. So, she called the police. And they fined the company for alteration of domar the. J.d.'s auto sales took that car back and refunded chekia for the full purchase price. We went back to j.d.'s to meet her salesman. How are you, man? Gio with "Good morning America." Let's talk about odometers real quick. He has no plans of talking on camera. A day before he told our producer by phone, we advertised it wrong. We changed the odometer because it wasn't working. I should have told her the odometer wasn't working. As for chekia, she already has a new car. Gio Benitez, ABC news, Atlanta. Great information to know. All cars have a unique vehicle identification number, a V.I.N., it's called. And the information is available online.
This transcript has been automatically generated and may not be 100% accurate.