Can that 2 wire harness REALLY cause my car to run rough like that? Or is it something else? If so, what?

I bought a 2001 Chevy Cavalier about a month ago and it ran fine. The check engine light came on a few weeks ago and I went to Advance and got my codes read(I have a friend who works there), well there was a P0440 EVAP code, so my friend checked the gas cap and it had condensation on it so my friend recommended I get a new one so I did, he cleared my codes, I had a friends dad check to see if he could find a leak on my hoses and he found nothing, well a day or 2 later the light came back on. Well I called my mechanic and he said as long as the car was running fine then everything should be fine, he said the smallest leak can set it off. So I went about my business. Well we went garage saleing this past Saturday and after a long day of driving we went home, then went to go to a friends house later that night, but the car wouldn't start, it would turn over just wouldn't start, so I called my friend who works at Advance and he said he would come over with a code reader in the morning, when he came over he told me to start the car so I did and it started but was running rough and very rich, had white smoke coming out the tail pipe and it smelled like gas majorly, so he scans my codes and the same P0440 EVAP code comes back, plus the coolant sensor code, but I knew that was going bad, my temperature gauge was acting wacky, so I knew it was a matter of time. So my friend check for leaks on my hoses with brake parts cleaner and NOTHING! So I decided to do some research on what it could be and on this website I found a lot of people where having the same problem and the culprit was a 2 wire harness that was on the passenger side by the gas tank. So I went and checked it and low and behold both wires were not connected at one end of the harness. I've already ordered a new harness and a new gas cap because someone said on here that a Chevy dealership mechanic did a smoke test and the auto zone gas cap they bought was garbage and Advance has gotten parts wrong with me more then once so I was just being safe. Well, I hadn't started my car in a day or so, so I figured I would the other day and it sounded like it was trying to die.