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Thread: The infamous P0421 code, need some help!

  1. #16
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    Quote Originally Posted by NCZ13 View Post
    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.

    NCZ13,
    Thanks for the response. If I go in the direction of sticking a nonfouler on the bottom O2, does that just eliminate the CEL for me? So in reality, I'm still driving around with a bad pre-cat and it can just completely die on me at some point while I'm driving?
    Where do I go about getting one of these race headers? Is is hard to install? Will there be any detrimental effects to my car with this race header? I have completely stock exhaust right now? Thanks in advance!

  2. #17
    Registered Member JimP02's Avatar


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    I have just started reading this forum, sounds like there's a lot of knowledge to be found,

    I have a 2002 p5, code problem multiple misfires, seems to run fine but check engine light keeps flashing......my mechanic says possible bad gas....any body point me in the right direction?

    It stutters once in a while, but it seems intermittent...reset itself once but now flashes continously
    Last edited by JimP02; 02-18-2008 at 08:35 PM.

  3. #18
    ......................... i12drivemyMP5's Avatar
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    I will list in order of expense but you should just do all 3 especially if the car has hi mileage and/or you do not know if they have ever had attention. plugs / wires / coils. Driving the car that way will cause more expensive problems.

  4. #19
    Registered Member JimP02's Avatar


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    Just had the plugs changed, less than 500 miles on new plugs, so I guess the next option is wires and coils......

    What damage might I cause?

    Thx for your quick response

  5. #20
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    Quote Originally Posted by JimP02 View Post
    Just had the plugs changed, less than 500 miles on new plugs, so I guess the next option is wires and coils......

    What damage might I cause?

    Thx for your quick response
    Hey Jim,
    Hopefully I can give you some insight on your problem. About 8-9 months ago, I was going through the same type of issues you were mentioning. At the time, I had about 90k miles on the car. I was doing the same thing...driving with the CEL on. It would come and go, with same code random multiple misfires...I left it alone until I no longer could cause the light started blinking instead of staying steady. I now regret trying to fix the problem earlier, hence my problem now with pre-cat going bad. Well, for me it was the coil packs. I replaced both coil packs and never had the random misfire again till this day, but I think due to the extensive misfires from the blinking CEL then... it has taken a toll on my pre-cat.
    Still waiting right now on answer about the non-fouler. And if anyone has any experience with the race header (which brand to go with and if installation is simple and direct bolt on) it would be greatly appreciated!

  6. #21
    mx3ownzj00 low_psi's Avatar
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    I've had to replace my o2 sensor a few times due to the P0421 code. My cat is just fine. The sensors went bad.



  7. #22
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    Quote Originally Posted by low_psi View Post
    I've had to replace my o2 sensor a few times due to the P0421 code. My cat is just fine. The sensors went bad.
    Did you have to replace both O2 sensors or just one of them? Why would you have to replace them a few times?! Shouldn't a new O2 sensor last you a good while? Are you sure it's your O2 sensors that are bad and not your pre-cat?

  8. #23
    Registered Member JimP02's Avatar


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    What would I notice if the 02 sensors were bad? Am I damaging the Cat by driving with the CEL light flashing?

  9. #24
    Registered Member JimP02's Avatar


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    Found out that My CAT is kaputt.......the P0421 and some tests indicate that the CAT is faulty.....

  10. #25
    手前の言葉が俺は聞き捨てよう TheMAN's Avatar


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    the problem causing your blinking "CEL" killed your cat, you should've stopped driving it when it happened because now you end up spending more money!
    TheMAN
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    For Protege and Mazda3 info, come see the Protege FAQ

  11. #26
    Mile High Zoom-Zoom Tom03es's Avatar
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    I've been keeping my primary cat nice and fresh on a shelf in my garage.
    2003 Laser Blue Protege ES-
    Mods: Injen CAI; MSP struts and springs ; Racing Beat rear sway bar + AXR Clunk Fix ; MSP STB; tinted windows; ceramic coated OBX header; AWR 88 durometer front & rear mounts and side mount inserts; MSP taillights; UR underdrive pulley; Racing Beat catback; 17x7" Enkei RSV wheels w/ 215/45-17 General Exclaim UHP tires; DaveB gauge rings; EBC Greenstuff front brake pads; Hayden transmission cooler; GT Spec 4 pt ladder brace; Medieval Motorsports alternator, water pump & power steering pullies; Autometer oil press, water temp, oil temp, trans temp gauges; VTCS-less intake manifold; Lentech Street Terminator VB; AutoExe lower arm bar; catted SLS mid-pipe

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  12. #27
    ......................... i12drivemyMP5's Avatar
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    Quote Originally Posted by Protege__Owner View Post
    Still waiting right now on answer about the non-fouler. And if anyone has any experience with the race header (which brand to go with and if installation is simple and direct bolt on) it would be greatly appreciated!
    OBX is direct bolt on with oem exhaust. Use 2 nonfoulers with one drilled out on the 2nd o2 sensor so no CEL.

  13. #28
    ......................... i12drivemyMP5's Avatar
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    Quote Originally Posted by Tom03es View Post
    I've been keeping my primary cat nice and fresh on a shelf in my garage.
    LOL, me too only mine's in a box for extra freshness.

  14. #29
    Mile High Zoom-Zoom Tom03es's Avatar
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    When I took my OBX header to get ceramic coated, they threw away the box. I had been keeping my manifold in that box. Oh well.
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  15. #30
    Backfires destroy MAF's!


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    Sorry to bump this old thread, but thought I would instead of starting a new one...

    My reasoning for posting/bumping instead of following/solving is that I read in a different thread that the EGR can cause this code?

    Here is the back-ground. Bought my early `02 P5 with 212,XXX miles on it about 3 weeks ago. CEL was on, previous owner had a reader and told me it was a bad knock sensor. No other codes to his knowledge, however, he had just purchased the car 12 days prior from a mazda dealership, so he really didn't know the history of the car...

    So, while waiting for a few things with the title work to be completed, I decided to clean the EGR since it seemed to be an easy task and the car had a slightly rough idle. In doing so, I accidentally took apart the sealed part. IDK what I was thinking, I was kinda out of it. So, did I mess up the EGR?

    Fast-Forward, knock sensor came in, turns out it wasn't even there! As in, I couldn't find the old one to remove because it wasn't there. Found a spot on the wire-harness where I thought it would plug into and it did. Found a spot on the back of the intake manifold to screw it into, done...

    CEL was gone for almost 30 miles, then back on. Thought I probably had the knock sensor in the wrong place, so I just decided to forget about it for the time-being...

    Replaced the plugs, installed the OEM intake, reset the ecu, CEL back on after 25'ish miles. Oh, and there was oil on the plugs/plug boots, bad valve cover gasket?

    Finally stopped by idiot-zone today and had them run the code. P0421. More information, if the car sits about 8 hours, she spits out some nice blue'ish white'ish smoke. Ok, a little more than "some" a lot! Oil? Or fuel?

    So what should I do in what order? I had planned to changed the plug boots, just haven't done it yet. Not sure if she's miss-firing on the interstate or if she's just really slow compared to my BMW. Would it be obvious if the car was miss-firing?

    Here is what I'm thinking...

    Plug boots (was going to do this anyway)
    EGR Valve (if it doesn't cure it, pull it back off and return)
    Fouler mod (seems cheap and easy)
    Replace cat (eek!)

    I'm confused that people have stated "coil packs" separate from "plug boots" I thought they were one in the same on these cars, am I wrong?

    ALL HELP APPRECIATED!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

    EDIT: Now that I've read almost every thread with "P0421" in the title, I think that miss-fire'ings could be the culprit. One poster mentioned some "bucking" by the engine, I feel this at times, just thought thats how she ran. Will most definitely replace the plug boots, still confused on "coil-packs." Please help! Since my cat is most likely plugged, I'll go ahead and start on the anti-fouler mod as well. Does running a plugged cat do any damage to the car? Or just the environment? I'll leave the EGR as the last option...

    EDIT II: Ok, so 2 have "coils" and two are "slaves." Correct?
    Last edited by imola.zhp; 08-19-2008 at 04:54 PM.

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