I sell one at Schucks that basically flshes the code at you. You just have to know what the code means. I think it tells you most, if not all in the book that comes with them, otherwise, you'd have to look in the vehicle manual to figure it out.
But for $35, it's alot easier than going to the shop to have the codes diagnosed.
Also, the cheap ones will only work up until '95. Anything newer and you'd have to spring for the $360 unit.
I have a full tank of Unleaded...
5 liters and 12 cylinders of fury....