I bought a small, handheld code reader from Sears that works on OBDII equipped cars from all mfrs. The codes are given on a software CD that comes with it. It allows you to clear the codes and the re-set the “Service Engine Soon” light, too. Even if you don’t have the device, Autozone will read (FREE!) the codes for you from their store owned device and give you a printout of what they mean......
Some codes...GM has a proprietary list they won’t share. I don’t remember which one stung me recently, but my regular mechanic had to take my Trailblazer to the dealer to obtain the definition. It is only going to get worse.