The infamous P0421 code, need some help!

bigyzf

Member
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...
 
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
 
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....
 
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
 
Yep, appt made at local garage that has the diagnostic machine to see what is defective. More to follow...
 
the "diagnostic machine" doesn't tell you s*** 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
 
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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.
 
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.
 
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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.

bigyzf said:
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.
 
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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.
 
Appt made with Mazda this week. More to follow..
Confirmed, cat ordered, will be replaced free!!!
 
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the "diagnostic machine" doesn't tell you s*** 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.
 
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.
 
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!
 
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
 
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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.
 
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
 
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!
 
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