service engine soon light

I recently bought I 1998 escort zx2. The car seems to run great but every time I put gas in the car the chek engine light comes on. I went down to autozone the first time it happened and they told me that the error was my car running to lean. He told me that it wasnt that big of a deal. He reset the light and it was fine until next time I put gas in the vehicle. Like I said the car seems to run good but its really annoying for the light to always come on. Any ideas of what is causing this problem. Thank you any help will be apreciated.