I've owned this 2004 Xterra 3.3 since September. And I've spent the majority of time working on it fixing problems. I thought I was buying something that would "run forever". Seems it will only "run forever" as long as you are fixing problems on it every other weekend!!! The last of my problems is this P0420 code. From my research this is most LIKELY either a cracked manifold or a bad cat. Either way it would require removing the exhaust manifold. I've removed the manifold heat shield and I don't see any cracks. I'm assuming the cat is bad. But it does run rough and I don't want to drive it until this is fixed. The bolts all seem to be fairly easy to access. But I know that the rust will be the bigger headache. How difficult is it to remove these manifolds (in a driveway without air tools)? How expensive is labor to remove this at a shop? Do you replace with stock components or aftermarket? If you replace with aftermarket, will there be any noticable problems leaving the factory exhaust on the drivers side (until it too goes bad)? This truck is SUPPOSED to be my daily driver. I am having to drive my weekend car until this is fixed. Which doesn't bother me except I want to keep the miles off of it and IT needs repairs done as well. Which i'd like to do once I get the Xterra running again.