What's wrong with my 99 ford f150?

I own a 1999 ford f150 xl 4.6 l. I had a service engine soon light on so I plugged in my obd scanner. It had one code, p0455. Which means major leak somewhere. But where could it be? I checked my gas cap its not that. Anyone know what I could check?

It could be any thing from the gas cap to a vacuum line. I finally found mine, and it was poor sealing of the gas cap.

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