Lol - did you know that weight can also affect the gas mileage. slim yourself up joking brother ... remove anything heavy in the car. small difference but a difference nonetheless.
if you could have remembered which codes come up you could have determined the problem by yourself. If you can get a chance to go see the seller and test drive it one more time ... you can note down the error codes and see what the problems are. anyways ...here is a complete compilation of the error codes for you
http://www.permoveo.ltd.uk/tabid/123/Diagnostics/Hyundai/Default.aspx