Wal Mart sells one i forget the price but it was under 50 bucks. most of the parts stores will loan or rent you one, some will even read it for you with the ones they have. i think someone said Pep Boys but it may have been someone else.
I haven't seen the one at Walmart, but I know Harbor Freight sells an OBD-2 code reader for $40-$50.
One thing I'll point out. The cheapest code readers typically only support generic OBD2 codes. They generally don't support OBD-2 enhanced or manufacturer specific codes.
Another point to make. The majority of the time, a DTC by itself isn't enough to "tell you what is wrong with your check engine light." A DTC is good for telling you where to begin your diagnosis, but can't do the diagnosis for you.
Of course, those kinds of functionality can be had, but at greater cost. If you do decide to go for the cheapest code reader available, recognize that you will be limited in how much diagnosis you can do, and you may still be dependent on someone else to get a good diagnosis.
I bought an Innova OBDII scanner for $99 from Pep Boys a few years ago and it has paid for itself many times over. You can get one off of eBay cheap. This one is less than $50: eBay Scanners don't answer all questions but they point you in the right direction. For more help read your DTC codes and either search these forums or create post.
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