2004 Nissan Sentra emissions leak

I am just trying to understand the problem with my car and the solution that was done to my car, 200.00 dollars later. A dump valve was put on a charcoal basket. I guess it needed replacing. Engine soon light came back on, I took it back to the guy that is dealing with it for me. Now I'm told not the right gas cap. Apparently clicking this one 3 times is not offering enough weight to not make it leak? Does any of this sound remotely correct to you?

I was not given a code. And I am not doubting the guy. He is pretty good. I am just trying to understand what he is doing. I had a hose replace this past year that had to do with my gas tank. He also informed me today that the return line is also new, because I asked if there could be a crack in the return line. He wants me to try another cap. He said what is happening is that each time something is done, a different number shows up, and they are going to try the cap to see if that works to keep the light off. If it doesnt, they are going to do"smoke thru the car" to see where it comes out so they can find the possible hole, if one. Does any of this make sense? They told me some of these cars are sensitive to emissions leaks. Even a pin hole. By the way, Thank you for responding.

O.K. Thanks.