The dealer I worked at was less than 1/4 mile from Autozone and those knuckleheads caused us no end of headaches. Half the time when they did their free scan they'd erase the memory so we couldn't complete the diag after they failed. The other half the time they'd sell the customer a part based on their mickey mouse code reader that didn't fix it. There are 2 levels of OBD II codes, generic and proprietary, only dealers can access the proprietary codes. The dealer scan tool, IDS, displays the actual codes and the tech then follows a diag tree to reach the actual fault. Often aftermarket scanners display the scanners guess of the cause. The one we'd see most often was "O2 sensor fault" popped up so they sold the customer an O2, he installed it and the light was still on. If the O2 is out of range it most often means the vehicle is running rich or lean, not that the sensor failed. It's a symptom, not a diagnosis. One used car dealer brought us an F150 after they'd replaced the O2 sensor 5 times trying to fix it using their scanner, my tech fixed it in 40 minutes. If you've unplugged anything, drive it for a few days and have a dealer diag it, it's better to pay for a correct diagnosis than to buy a bunch of parts based on faulty "free" diag.