ok , I just wanted to ask if anyone knows why or what causes a trouble codes, in regards to ur fuel cap/fuel door or filler neck???
thanks

I'm not an expert, but the fuel tank periodically gets silghtly pressurized to let the diagnostic system know if there is a big leak or hole that lets gasoline vapors get out of the emissions system. If the system does not pressurize properly, it throws a code. If you leave your gas cap off, or if the seal on the gas cap is not sealing, or if there is a hose broken or pulled off, it will throw a code.
It is really just a system to tell you that your emissions system has a leak someplace, I think. I'm sure the mechanics on here can explain it better than me.

I too seem to be having a codes problem and it seems related to the gas cap. I replaced it a year ago which fixed the problem but am getting the PO420 again. It seems to only come on right after I fill the tank with gas and sometimes the codes clears a week or two later. Has anyone else had this problem?