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post #1 of 4 (permalink) Old 08-27-2011, 04:26 PM Thread Starter
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Infamous P0421 Code

Good day all. I have an '04 6s with 110k miles on it. I've had the car since 05' bought from a dealer in NY with 9k miles on it. Never had a stitch of problems other than a coil here a fresh air intake there. However, recently I have experienced the nasty P0421 code. I've read almost ALL of the posts pertaining to this issue and still do not have a solid understanding. I have NOT replaced any of the O2 sensors in the car since purchasing it. Should I change the O2 sensor before jumping on the precat replacement bandwagon? Here are my questions in addition to the above.

1. Precat issue 0421 is bank 1 or 2 AND which is bank 1? Front or rear?
2. I plan on keeping the car and giving it to my son who is starting college. The car is completely stock. Should I just replace the precat and if so would you recommend replacing the other one while I'm under there?
3. If I were to put MSDS or CP-e headers on the car what does that do to the O2 sensors and CEL? I live in NC where they do OBD emissions tests.
4. It appears in all the posts that I've read that the header installation is relatively easy. Does the install go from the headers to the single center cat?

Thank you for your input and apologize for the redundant question.
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post #2 of 4 (permalink) Old 08-29-2011, 01:32 AM
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msds w pre cat and you should be okay or gut your current cats and maybe move the position of the o2 sensor.
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post #3 of 4 (permalink) Old 08-29-2011, 05:49 PM
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Quote:
Originally Posted by aguacci@yahoo.com View Post
1. Precat issue 0421 is bank 1 or 2 AND which is bank 1? Front or rear?
2. I plan on keeping the car and giving it to my son who is starting college. The car is completely stock. Should I just replace the precat and if so would you recommend replacing the other one while I'm under there?
3. If I were to put MSDS or CP-e headers on the car what does that do to the O2 sensors and CEL? I live in NC where they do OBD emissions tests.
4. It appears in all the posts that I've read that the header installation is relatively easy. Does the install go from the headers to the single center cat?

Thank you for your input and apologize for the redundant question.
1. P0421- downstream bank 1.
bank 1 is the rear bank

2. don't jump until you have exhausted checking other things.
3.very few have had MSDS or CPE without a code. Most will override the sensors to eliminate the codes.
4.define easy? easier than some, but I have done it and could probably knock it out in about 6 hours with my well stocked garage.....
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post #4 of 4 (permalink) Old 09-24-2014, 11:39 PM
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Quote:
Originally Posted by aguacci@yahoo.com View Post
Good day all. I have an '04 6s with 110k miles on it. I've had the car since 05' bought from a dealer in NY with 9k miles on it. Never had a stitch of problems other than a coil here a fresh air intake there. However, recently I have experienced the nasty P0421 code. I've read almost ALL of the posts pertaining to this issue and still do not have a solid understanding. I have NOT replaced any of the O2 sensors in the car since purchasing it. Should I change the O2 sensor before jumping on the precat replacement bandwagon? Here are my questions in addition to the above.

1. Precat issue 0421 is bank 1 or 2 AND which is bank 1? Front or rear?
2. I plan on keeping the car and giving it to my son who is starting college. The car is completely stock. Should I just replace the precat and if so would you recommend replacing the other one while I'm under there?
3. If I were to put MSDS or CP-e headers on the car what does that do to the O2 sensors and CEL? I live in NC where they do OBD emissions tests.
4. It appears in all the posts that I've read that the header installation is relatively easy. Does the install go from the headers to the single center cat?

Thank you for your input and apologize for the redundant question.
Just fixed this issue on my wife's car. check out my thread below.
Good Luck!!

P0421 and P0431 code

03 Mazda 6s
3.0 V6
5-speed
Magnaflow Headers and CAT system
AuxMod cable
LED interior lighting upgrade
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