Hey guys, so I have on '06 with the 2.7L engine. I lopped off the stock exhaust just behind the 2-to-1 "y" pipe and welded in 3 inch the rest of the way back. My car didn't immediately show the code but eventually it did, P0420. I thought, "ok, no big deal. Just swap out O2 sensors and be done with it." That worked for about a month and the code came back. No big deal, car seems to run fine and I have a few months to work the problem out before emissions rolls around. So, I ordered new cats (the company said they were eom quality) same pipe size as my factory cats... It didn't fix the code. I took it to a shop and paid their $350 diag fee so I can figure it out. They said my new O2 sensors were low quality and that should fix the problem. Ok, I bought higher quality sensors and installed those. That worked long enough for my car to pass emissions and LITERALLY when I started my car after emissions passed and I'm getting ready to leave... CEL turns on. I'm at a loss here. What could it be?