All this started with a cat efficiency CEl that would mostly only come on at highway speed or high speed in general, I could clear the code, and it wouldn't come back on until I started going 60-70 again. Often being a month or so in between. I finally decided to replace the post cat 02 sensor on both sides (despite the CEL only showing bank 1). After that the one code turned into two showing that the system was too lean on both sides and still only comes on when i get to highway speeds. I've replaced one of the pre cat sensors so far and in the process had to get new gaskets for the header and cat due to everything going wrong, but it still comes up for both sides. I'd prefer not to have to buy new cats but apt I don't want to buy them and it not be the problem (I'm broke). I've yet to do the fuel trim relearn to see if it helps, but im a little skeptical as the only reference to it is a single comment on one thread. If there's any other solution I'm missing please lmk!!