Help need to know were to start looking

Well my CEL came on last week so i went to autozone to check it out and they said that i had a small emissions leak (error p405 i think) then they ask me did i just get gas cause it might be my gas cap not on tight, ok they erased then 2days later it came back on same code , then this time they said that it may be a vaccum line leak, they also said since its small dont worry about it ,but this is a intrigue and u dont ignore problems or they come back big. anyway just wanted to know were should i start looking for the problem ,again its a small emission leak. my cel light tends to annoy me. AS ALWAYS ALL HELP IS GOOD HELP THANKS IN ADVANCED.

I would inspect all the vacuum lines. Then I would check your PCV valve. It is on the front valve cover if you have the 3.5L engine. Remove it and make sure it moves freely by shaking it. Replace it if it does not. Other symptoms of a bad PCV include excessive oil consumption and a decrease in fuel economy. If that checks out OK then you may have a more serious (but not "leave you stranded" serious) problem. The next thing I would check is your fuel pressure regulator, especially if it seems to need to crank longer to start after leaving it sit a while. The regulator is located on your fuel rail and has one vacuum line going to it. Remove the vacuum line and if you see gas then it is probably bad. The gas cap typically throws a P0440 code so I doubt it is that.