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p0421 Code after plugs/wire/coils

Old Jun 21, 2018, 03:45 PM
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Question p0421 Code after plugs/wire/coils

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.
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Old Jun 25, 2018, 03:06 AM
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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
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Old Jun 26, 2018, 12:35 AM
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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.

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