The infamous P0421 code, need some help!

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.
 
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?
 
What would I notice if the 02 sensors were bad? Am I damaging the Cat by driving with the CEL light flashing?
 
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!
 
I've been keeping my primary cat nice and fresh on a shelf in my garage.
 
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.
 
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.
 
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?
 
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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.
 
^ 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...
 
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*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.
 
^ 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?
 
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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.
 
Mazda 3 p0421

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
 
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