New to the Forums. Ive been reading and researching for a while now. I used to have an 02 spec v. This site helped me through my butterfly issues. So I just purchased a 2006 ser 6mt about 3 weeks ago. Ive had nothin but problems. I will start at the beginning. The car had everything from the stock y pipe back removed and straight pipe put in. I have 2 kids and although it sounded good when driving it just wasnt practical. So i put the stock mufflers back in there hangers and all was good. I started to check the engine codes. I was getting p0158 and p0057. So i changed out bank 2 sensor 2 for a new one. Now i was having a problem where when i go WOT the car starts to pop and misfire quite badly at around 5000rpm and such bad hesitation that it wouldnt accelerate any more and the engine light starts flashing. So i started goin over the car the best i could. The car has a k&n short ram and the vais removed. So i checked front coils and plugs there good. So i removed the intake manifold to find that the left most nut with stud area had broken completely off(thanks to the previous owner. I asked if there was any issues i should no about). Anyways i bought a new intake from a junkyard off a 04 quest. Now while the intake manifold was off i switched the coils and plugs front to back just incase. Put it all back together. And still same popping hesitation sound and terrible misfire. Ive cleaned the MAF. Ive cleaned the injectors and gotten better fuel milage. Now the misfire is getting worse and im pretty lost with this one. I took it to a local mechanic and tire shop where my brother works and Put it on a very expensive scan tool. Maf is working 100% and the o2 sensors are working correctly except that one i put in is alittle wild on the charts when monitored. Now through all the bs the car started working Really really good. Ive owned a 02 maxima 3.5 an i new what to expect performance wise. Now i reset the battery to get the sunroof to work and presto its back to poor acceleration misfire at 4000rpm and popping like the intakes come off. I put it back on the hoist for further inspection of the cats. Ive been reading alot on these forums and i no there notorious. Now my rear cat has cracks at the bottom of it and the previous owner did a piss poor job of welding it. So now could this leaky or pluged cat b causing all my troubling misfires. Please help i dunno what else to do other than buy the RL hI flow cats an y pipe. I just dont wanna spend a $1000 for it not to fix. Codes for misfire is p0300 and p2000. Im not having hard start so i eliminated the crank sensor.