The OE scan tools can read the codes, reflash the PCM as needed, as well as display live data readings from the sensors, fuel trims, misfires, etc, record the data, as well as allow it to perform a variety of tests. It wont tell anyone what to fix, or how to fix a vehicle, but it will give a slew of info to the technician, and allow them to find the problem. With just a misfire, it could be a matter of a bad coil, or spark plug, or something else. Scan tool wont always help with that, but the diagnostic time you are paying for should cover them to check the coil, etc. A popular misconception is, you bring it to me at work, I charge you $96, or whatever the going rate is in your area, and I hook up the scan tool, and presto I have the answer. Sadly, some mechanics think like that too. Reality is, it comes in, I hook up to it, and drive it, and watch the various readings. From there, depending on the problem, I do whatever tests are needed to find the problem. You can buy the factory service manual, and it will usually get you thru problems, but the factory test procedures are based on unaltered vehicles, in near new condition, and far from the reality of a car thats 5 years old or more. Lots of years of experience will teach you where you need to go with the problem at hand, and save time and money. GM is well known for "replace with known good part" for test procedures. Yeah, right,lol.