My Hyundai Elantra has a check engine code p0451, what do I do?

I tightened the gas cap, but it came back. I know it could be a bad seal on the cap, but I just want to know what else I would have to look into and how do I find out what is really wrong? I don't want to have to start buying a bunch of parts to do the trial and error thing. So what is the best way to do a process of elimination thing?

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