[legion]

I have a $10 Amazon OBDII scanner.

It's perfect for my purposes: It let's me know if the problem needs to be taken to a professional or if I can fix myself. Once I have the code, I get online and research the causes for my particular make/model.

So far, I've used it once. My wife's minivan had a check engine light come on. The code description that came up was something like "evaporative emission system failure". A little research showed the most likely cause was a loose gas cap. I also learned that the check engine light would continue to come on until the code was cleared. I tightened her gas cap and cleared the code. That was a year ago. It saved me the time/expense of having a professional look at it, so it has already paid for itself with one use.