Gosh Darn Engine Light

I recently got my 350z 2004 about 2 weeks ago. When I got the car after driving about 50 miles my engine light went off. When taking my car to auto zone they printed out a slip that said something in my gas tank temp gage is defective. When I went back to the place to get it replaced, after driving about 300 miles the light has came back on. What is going on? Does anyone else have this problem or know anything about it, and is it a major problem or is this something I need fixed right away?