I did a little fix up on my car (03 Lancer OZ Rally), as a friend is letting me use their much better MPG vehicle for a road trip and using mine (and mine has been running like a grandma due to AC/the 234K miles on it). I replaced my plugs with the iridium plugs, new boots, coils, a new air filter as the old one was nasty, and I topped off oil due to a slow leak. I ran a few errands, and the car is running smoother and I am not feeling as big of a drag on power with AC as I was. However, when I got in after my last errand, the CEL was on. Torque said it was error p0421.

Any ideas? I cleared the CEL, and I'm assuming it was simply calibrating everything from the mini-tune.

 

you need a new cat converter... changing plugs/coils/wires wont do ****..
the one that's failing is oem header that has the cat built in...

if you live in a epa emission controlled state you need one that's 50 state legal, otherwise, their is a federal one that's good for 48 states ( except CA and NY )

I had to replace mine that I had same idiot code as you

 

here is a visual reference of what I am talking aboot
here is what I did to fix my p0421, I had to replace this part... on right is OEM and on left is Magnaflow Stainless Steel Part that's 50-state legal.

 

I had the same code. I replaced both exhaust manifold w/ built in CAT and the catalytic converter located under the vehicle. For the exhaust manifold, I replaced it with the OEM updated one as the original exhaust manifold had a recall or tsb on them and I replaced the cat under the vehicle with Magnaflow (CA Approved). Three months later and no error code

Here are the graph examples (before & after) of a bad and good CAT

1st Graph: Bad CAT= Bank 1 Sensor 2 (O2 sensor under vehicle) imitates Bank 1 sensor 1(O2 sensor under hood)

2nd Graph: Good CAT= Bank 1 sensor 2 (O2 sensor under vehicle) is constant as Bank 1 sensor 1 (O2 sensor under hood) fluctuates normally

 

I have this same issue with another car and I've been trying to sort it for months. I can't pass smog due to the issue P0421 ( Warm Up Cat Efficiency Bank 1 Below Threshold ). The cheapest fix for the other car I have and is temporary is to use an 02 spacer on the downstream sensor. They cost like $8 for one and it throws off the reading and is not the proper fix but works for the car I have supposedly. If you want to fix it the right way, doing the above fix mentioned is best option, You might also be able to go after market header etc.