Always happens that way. Once you mod it's a never ending game. Give up now. lol
As far as O2's go I'd highly recomend going with a factory sensor. It really can be a hit or miss in the afermarket world especially when they are half the price. I've seen customers come in claiming they replaced it twice and still getting a code. After pulling the code and reading the data they still needed a sensor. Put one in and no more issues. With that said you can get lucky and get good cheap sensors as well. You just never know if they are good or how long they will last.