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Thread: The infamous P0421 code, need some help!

  1. #31
    Registered Member P-Funk!'s Avatar

    Lancer Ralliart Sportback

    Join Date
    Jul 2002
    Chesapeake Beach, MD
    Okay. You have four plug boots. Two are directly on the coil packs that sit over two cylinders. The other two cylinders are slaves and have spark plug wires from the coil to the plug. If you order plug wires and new coils - they come with the boots.

    Oil leaking into the plug wells means the valve cover needs to be removed and the seals replaced.

    Yes, you probably need to replace the EGR as well. It may be working now - but by breaking the seal - you introduce corrosion and future failure.
    I used to have a
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  2. #32
    Backfires destroy MAF's!

    Join Date
    Jul 2008
    At your mom's
    ^ thank you for the help. Each "auto-parts" store web-site makes it VERY confusing to understand what you need and what you get for what $$...

    I stopped by Advance on the way to work this morning to pick up a couple of 18mm anti-foulers. I thought I'd be able to put them on here at work today, but I dunno, not if the boss doesn't leave soon, grrr...

    Further inspecting and trying to understand the plug wire/boot/coil situation I realize that one of the slave wires is taped up with black electrical tape. LOL That can't be good!

    Also, as I started the car to leave yesterday, she was TICKING like a mo-fo. Reminded me of my BMW after an auto-x run on not completely full oil. Got home, pulled the dipstick and it was nearly dry. haha I love beaters, poured in 1/2 a quart and she was all good, I'll re-check it again when I get home tonight to see what she needs. When installing the knock sensor I noticed it looks like the oil pressure sensor is leaking oil. The bottom oil pan might be too...

    I've got a lot of work to do on this thing...
    Last edited by imola.zhp; 08-20-2008 at 02:21 PM.

  3. #33
    Registered Member P-Funk!'s Avatar

    Lancer Ralliart Sportback

    Join Date
    Jul 2002
    Chesapeake Beach, MD
    *Plug and coil wires have a black plastic protective wrap to prevent chaffing - is this what you are talking about?

    *If you replace the cat - you do not need the anti-fouler.
    If it is running rich/missing - the cat is bad and it needs to be replaced anyway. You only need the anti-fouler trick if you install a header.
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  4. #34
    Backfires destroy MAF's!

    Join Date
    Jul 2008
    At your mom's
    ^ not that coating, this is actually black electrical tape. This car has some "issues."

    Well, I pulled the o2 sensor, cleaned it with a wire brush, drilled out one of the 2 anti-foulers, put it all together and so far have been 59 miles without a CEL!

    That is the furthest ever! Previous record was 28, lol, record...

    This "fix" is just temporary until I find a permanent solution/have the cash for the permanent solution. I just don't like cruising around with a CEL...

    Taking a big vacation next week, when I get back I'll first replace the plugs/wires, then see how the cash flow is for a new/used cat or a new/used header...

    Thanks again for the help!

    EDIT: Hey, so since this "Trick" worked, that would most likely show that the cat is indeed plugged, right? I'm going to replace the coils/plug wires/boots just because it probably needs done and the one has electrical tape on it, but yeah, wouldn't you say that since this worked that the cat is most likely plugged? Or could there be other "issues" as well?
    Last edited by imola.zhp; 08-21-2008 at 10:15 AM.

  5. #35
    Registered Member thrasher's Avatar
    Donated: $20

    '02 Protege5

    Join Date
    May 2006
    Ontario, Canada
    When the coil pack starts to go it will throw a P0421 code, usually on the highway when hot, and the car will barely run. It will smell like sulfur (eggs). After restarting the car it will run just fine for a while until it does it again. Coilpacks are easy to replace.

    I also threw the code for precat below threshold... I was beyond the warranty so I took the internals out of the pre-cat and used a non-fouler. I haven't had any issues since. No CEL's at all.

    This may not be the best way to resolve the issue, but it certainly fits my budget and it's no worse for the environment than running a header.

  6. #36
    Registered Member

    MAZDA 3

    Join Date
    Mar 2019

    Mazda 3 p0421

    Quote Originally Posted by bigyzf View Post
    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...
    this is caused by a sticking thermostat causing the engine not to warm up fast enough

  7. #37
    The Diagram Dude pcb's Avatar

    2002 mazda protege 5

    Join Date
    Jun 2011
    ontario, canada
    Quote Originally Posted by sparkybuoy View Post
    this is caused by a sticking thermostat causing the engine not to warm up fast enough
    I'm sure I could remove my thermostat and my pre-cat would still warm up.

    Quote Originally Posted by TheMAN View Post
    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!

    The Diagram Dude

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