In this video I go over how I found the true cause of a p0421 code, I also done this on my 8th gen 2003 Mitsubishi Galant and on a few friends cars too, it seems to be a common problem on 1999-2005, 4g64 2.4 Mitsubishi engines, with the front catalytic converters, a lot of people just mask this problem with a spark plug fouler/spacer or end up spending lots of money on a new catalytic converter when the old substrate is perfectly fine mine was I inspected it when I removed the unit from the car, and welding the crack has solved my code, I had to prep the manifold by grinding a channel over the crack and drilling holes at both the ends of the crack so it would not be encouraged to spread after the repair.
