I finally changed the plugs, wires and new valve cover gaskets on the 2001 Montero Limited (full-sized). When I got it back together, it was or is running like crap. It has a rough idle and the check engine light is on. I ran my scanner and these two codes appeared p0400 and p0421. I have checked everything and it appears to be right. I do hear a slight sucking noise. I code advised cat. converter problems and the other egr flow malfunction. Any advise is greatly appreciated!! Thanks!!!

 

Definitely something you did. Break it all down again and inspect/ test all new parts or replace with the original to see if problem goes away. Definitey dont chase ghosts - focus on what you did. Did you bump a vacuum hose or wire out of place?

 

Did you unplug the battery? The "old" maps are referrenced and that could also be causing the rough idle.
The sound of air is not good, so definitely backtrack and make sure you have the valve cover bolts torqued properly. Make sure you reinstalled all the vacuum hoses, piping, wiring, and check around your EGR. The cat and EGR are obviously two different issues but relate to one another downstream.