Howdy folks. I just wanted to add a little to what jdeal was saying. Yes, there is a HUGH difference between readers and scanners. And yes, one can easily spend upwards of $5K on a good scan tool and all the related attachments. And yes, the Auto X-ray is a good tool for the hobby mechanic, and probably the best "bang for your buck" option. However, if you use them everyday, the Auto X-ray does lack in some areas, which I won't get into. The main differences have to do with the amount of accuracy and the frequency of sampling. The Snap-On and OTC scanners are my personal favorites.
Anyway, first of all, the guys at the parts counters are not technicians/mechanics, and trusting their opinions can easily lead you down the wrong road. I do have to defend myself and other REAL techs here though. When someone is asking me a question, there are always at least 15 things that pop into my head on what the related problem could be. Code readers and also code scanners are tools, and nothing else. They can lead you in the right direction, but definately do not tell you what is wrong. You have to get in there to find out. Like jdeal was working with his TP sensor. Just because you got the code, did you know it was a corrosion problem? Did the scanner TELL you that? Could have been a bad wire, bad sensor, bad harness connection, or bad ECU, etc. My point is that a tool is only as good as the tech/mechanic who is using it. (Jdeal I'm not picking on you, only using your example as my own, no offense)
When you do go to a shop just remember, you are paying for the training, expertise and experience of your mechanic. Not to mention the enormous cost of equipment, insurance, etc. Think about how difficult it is when someone says, "I've got this funny noise, it's kinda a growl, whizz noise coming from the inside of the motor. What is that?" And then the customer wants you to look into it for free or gets mad when you charge them for diagnosing and not repairing the problem. Time equals money. And nothing enters and leaves my shop for free, just like the doctor's office. If you're looking for a more economic alternative, look into your local community college or high school automotive programs.
-Kevin