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Thread: The infamous P0421 code, need some help!

  1. #1
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    The infamous P0421 code, need some help!

    Hey all,
    I have been doing alot of searching on this topic and im not sure how to proceed. Here is the deal.
    About six months back I had ignition problems that were hit and miss on my 2002 model, all stock. And the CEL code then was cylinder misfire. Over time I eventually changed the plugs and ignition coils and it fixed the problem.
    But my check engine light is still on reading the P0421 code now "Cat below threshold during warmup".
    Question is: Is the cat damaged from the misfire?
    Should I change the O2 sensors?
    I inspected all the harnesses on the O2 sensors for loosness, rechecked the plugs for fouling and also there is no exhaust leaks detected. I have reset the CEL a few times by dissconnecting the battery for a few min. 40miles later its on again with same code.
    Im stumped and I dont want to take it to Mazda to rape my wallet. Car runs perfect and gets 30mpg... But im at 77k and the emissions warranty is through 80k i believe. Any ideas are greatly appreciated...

  2. #2
    The Pitbull Fan dwpeek's Avatar

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    Check some of the other threads having to do with code. O2 sensor of EGR valve. There are alot of thread on this matter. Or to simpify, check the FAQ
    I GOT A MOTOR!!!

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    Yea i have searched until my eyes are red..
    What is the deal with this non-fouler method i see? Does it keep the CEL from coming on at all? And where can i get the parts for this install and instructions?
    Or perhaps should i just buy a new O2 sensor? Car runs great...

    Help....

  4. #4
    手前の言葉が俺は聞き捨てよう TheMAN's Avatar


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    dude, it's exactly what the code says... the cat is bad, the car is under warranty, stop wasting time and bring it in and get it replaced FREE!

    yes, misfires can kill cats
    TheMAN
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    For Protege and Mazda3 info, come see the Protege FAQ

  5. #5
    The Pitbull Fan dwpeek's Avatar

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    Quote Originally Posted by TheMAN View Post
    dude, it's exactly what the code says... the cat is bad, the car is under warranty, stop wasting time and bring it in and get it replaced FREE!

    yes, misfires can kill cats
    That's why he's THE MAN.
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    Yep, appt made at local garage that has the diagnostic machine to see what is defective. More to follow...

  7. #7
    手前の言葉が俺は聞き捨てよう TheMAN's Avatar


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    the "diagnostic machine" doesn't tell you shit about what's defective, it only tells you what you already know... there is NO magic to automotive repair... you can't hook up a computer to the car and expect it to magically fix all problems nor can you expect it to pinpoint exactly what is wrong with it

    I told you, your cat is bad... stop wasting time and money and go get the cat replaced under warranty! your misfires is what killed the cat, and furthermore, the diagnostic systems for detecting a bad cat is 99% accurate compared to other systems in your car... there is NO need to waste time and doubt what the ECU is telling you because of this

    replacing O2 sensors and other parts = waste of money because they're still doing their job and the ECU is telling you what's wrong with the car!
    bringing it to joe's auto repair = waste of money because they're going to charge you to look at the car only to find what I've already told you... the cat is bad!!!
    bringing it to the mazda dealer = SAVE MONEY because this specific problem is covered under warranty! there is NO "rape my wallet"
    wasting time with all these superficial diagnostic and repair attempts = waste of time and money because the next thing you know, your car has more than 80000 miles and you're no longer under warranty
    Last edited by TheMAN; 01-12-2008 at 05:30 PM.
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    Registered Member brett02's Avatar


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    I have to agree with "theman" I researched the same thing that you are having problems with and if my car didn't have 86k on it I would have gladly let Mazda put a new cat on it. Yours is under warranty, take it to Mazda and have them replace it.

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    Problem is everybody wants to put the machine on it to see what is wrong. Yes this was done at autzone but nobody will touch the car until they put their computer on it. Its aggravating but nonetheless I have an appt with a shop next week and I will argue the case that I have the code so order a new cat for me. We will see how it goes. I hate dealing with auto repair places myself they just piss me off...

    I will bring this with me:

    If you have valid warranty claim, you cannot be charged for any costs associated with the diagnosis or repair of the problem, including labor charges, parts, or miscellaneous items that are necessary to complete the repair. For example, if a manufacturer agrees to replace a catalytic converter under the emissions warranty, you should not be charged for the diagnosis of the bad converter, or any pipes, brackets, adjustments, or labor needed to complete the replacement.
    Last edited by bigyzf; 01-13-2008 at 01:30 PM.

  10. #10
    Registered Member memo79's Avatar
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    Did you read the last post TheMan put up?
    I'm not trying to be an ass here, but I'd do the following:

    Listen to what TheMan says.

    I'll simplify:

    1. Cancel your appointment with whoever it is you have it with.
    2. Take it to Mazda dealership.
    3. Enjoy your new cat.

    Of course Mazda is going to put their Diagnostic machine on it. They want to know what the problem is. DON'T tell them you know it's P0421 or whatever. Just let them do their job. They will see what it is and fix it.

    Quote Originally Posted by bigyzf
    I hate dealing with auto repair places myself they just piss me off...
    The real reason your getting pissed off is 'cause you keep going to the wrong places. Go to Mazda.
    Last edited by memo79; 01-13-2008 at 01:51 PM.
    (p5black)

  11. #11
    The Pitbull Fan dwpeek's Avatar

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    Quote Originally Posted by dwpeek View Post
    That's why he's THE MAN.
    I think it was me who said, "that's why he's THE MAN".
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  12. #12
    Registered Member brett02's Avatar


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    Take it to the Mazda dealership, they will read what the code is under warranty and fix. I know I am a newbe here, but I just got done dealing with all of this my self.

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    Appt made with Mazda this week. More to follow..
    Confirmed, cat ordered, will be replaced free!!!
    Last edited by bigyzf; 01-16-2008 at 12:35 PM.

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    Quote Originally Posted by TheMAN View Post
    the "diagnostic machine" doesn't tell you shit about what's defective, it only tells you what you already know... there is NO magic to automotive repair... you can't hook up a computer to the car and expect it to magically fix all problems nor can you expect it to pinpoint exactly what is wrong with it

    I told you, your cat is bad... stop wasting time and money and go get the cat replaced under warranty! your misfires is what killed the cat, and furthermore, the diagnostic systems for detecting a bad cat is 99% accurate compared to other systems in your car... there is NO need to waste time and doubt what the ECU is telling you because of this

    replacing O2 sensors and other parts = waste of money because they're still doing their job and the ECU is telling you what's wrong with the car!
    bringing it to joe's auto repair = waste of money because they're going to charge you to look at the car only to find what I've already told you... the cat is bad!!!
    bringing it to the mazda dealer = SAVE MONEY because this specific problem is covered under warranty! there is NO "rape my wallet"
    wasting time with all these superficial diagnostic and repair attempts = waste of time and money because the next thing you know, your car has more than 80000 miles and you're no longer under warranty
    The Man,
    I would greatly appreciate some of your knowledge and guidance on this CAT issue. I currently drive a 2001 Protege ES with 95000 miles on it. I have thrown this same code P0421 twice already in two months. It was on for about 3 days, went off... and a month later, yesterday, it just came on again. Based on what you have said, I definitely have a bad CAT? Driving still seems perfectly normal, nothing unusual with the light on. I do notice occasionally when at a red light, the idle drops down a little close to 0 and then jumps back up to normal, a little below the 1.
    Now I'm guessing because I already have 95000 miles on the car, it is no longer under warranty right? If i wanted to replace the CAT myself, is it a DIY job or definitely an experienced mechanic job? I believe their is two CAT's on my car. Which one would be giving me this P0421 code? Thanks in advance.

  15. #15
    Registered Member NCZ13's Avatar


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    its your precat that is bad.

    when a cylinder misfires, not all the fuel is ignited, causing the fuel to travel down the exhaust system through the cats. this is what damages the cats. the unburnt fuel.

    whats giving you the code is your precat. being that the secondary cat has no o2 sensors after it. you can either A) stick a nonfouler on the bottom o2 sensor and kill walruses in alaska with your poisonous gases, or b) you can get a new stock manifold. but if your going to do that you might as well get a race header that eliminates the cat altogether, and gives a the car a nice boost in power in the mid to top end.

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    http://www.mazdas247.com/forum/showthread.php?t=98864

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