'07 Suburban p0455 but "no" leak

[HARLEY'S LIMO]

Hi. I am new to this, so please be patient with me. I have a 2007 Suburban that recently threw a po455 code after my check engine light came on. I changed the gas cap and resest it only to have it come back a cou ple days later. This continued for a few days. My low tire pressure light then came on. All tires were properly inflated. I still added a couple psi to even them out but could not make the light go out. The obd now shows both a po455 and po496, but nothing about the tires. Could this possibly be a computer malfunction? Any suggestionswould be greatly appreciated, as I can not afford a pricey mechanic bill...
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