P0421 on 2002 626

[CrazyTasty07]

I'm getting a P0421 on my 02 626. I've been trying to get an emissions test done and can't seem to get this code cleared. When I originally had the test done, I was getting a P0300 and the P0421, however, I've since had the plugs & wires changed, and replaced the O2 sensors. The P0300 is gone, however, the check engine light has come back on and I'm still getting the P0421. I have no knowledge of car repair at all, but I was wondering if anyone might have any ideas as to what else we can try fixing?

 

[Fudgie]

What do they do for an emissions test? Is it an OBDII scan or an actual tailpipe test? The P0421 is a catalyst efficency code and most likely the cat will need to be replaced. Its failure may be the result of the misfiring that you have already repaired. Is it a 4cyl or V-6?