There are many many diagnostic tools on the market and all range in price some very cheap and some very expensive. If you but really cheap ones then don't expect to much from them. If you were going to use them on a frequent basis then it would be prudent to go for the better quality ones.

Otherwise if it is just for a fault on your car then a garage with good quality equipment may be the answer. Another thing to note is that if it is for your present car,then it is unlikely that your OBD is functioning and so it is not worth purchasing any diagnostic.