There is no reason to think that your tool doesn't read the same stuff as the mechanic's tool reads. The codes are either OBD I or OBD II, depending on the year of your car, and the codes generated by OBD II are standardized just so people like you and I can buy the tool to read the codes and save money on repairs. My guess is you have a car that is 1996 or newer, and uses OBD II because there is no code reader to read OBD I. (The qualification being that the tool needed to read OBD I is expensive, and most home-mechanics won't have one in our collection of tools. If you actually have one, the odds are it's the same one the mechanic has, and you will have paid considerably more than $150 for it.)