How do you replace the catalytic converters on the qx4?

Well it depends on the model, but they are all mostly the same, and not to hard to do. Your only looking at about 4 bolts, and 2 nuts. There are 2 upper bolts (13mm) 2 rear bolts, with nuts on the back side (also 13mm, but the nuts are usually 12mm) Then 2 more nuts in the middle, supporting the exhaust to a bracket. You will need new seals for the new exhaust, and I also highly recommend getting 2 new upper bolts. One side can be easier then the other, because it has a 'butt-joint' where it connects to the rear portion of the exhaust, the other side has a 'slide-joint', and usually requires the other side to be loosed to help remove. Also remember that there are 2 oxygen sensors that will need to be unplugged and transfered over to the new exhaust. (22mm) I would start with removing the 2 upper bolts, next remove the 2 rear bolts and nuts. Then remove the side nuts last while supporting the exhaust. then the exhaust can be removed down at the rear and out. Good luck, anymore questions....just ask.