P0421 Mazda Code - Warm Up Catalyst System Right Efficiency Below Threshold

97 Mx6 v6...runs excellent until operating temp reached...then stutters under acceleration

There are many things that can cause that problem, but since you have a P0421, it may indicate that there's a problem with the catalytic converter. If the cat is damage, it could restrict the exhaust flow and make the engine stutter.

I have the P0421, but no engine stutter. My car is California emissions, so I have 3 O2 sensors, a pre cat, and a cat.....could it be the pre cat, or the main cat?

What model and year is your Mazda?

2008 Mazda 6i

I'm assuming that you have a V6 engine. If so, the P0421 refers to the Warm-up Catalyst (Pre-Cat) on the right bank. On your Mazda the pre-cats are part of the exhaust manifold.

I drive a 2007 Mazda 6, 4-cyl, CA Emissions with over 150,000 miles (I bought it with only 39,000). It still runs perfectly, but my Check Eng light has been going on and off for probably a year now. It will stay off for days and then come back on for days. Anytime I've had it scanned, it only shows P0421 code. Does anybody know if it might only be a sensor or will I eventually have to replace one of the cats? Thanks in advance for any help.

Unfortunately you may have to replace the cats.

I have a 2005 mazda 3S 2.3L with about 140K for about a year now and its coming up for inspection and I have this p0421 code. I have changed both O2 sensor and cleared out the code, once I've gone thru the drive cycle it pops up again. Im assuming I have to replace the CAT.
Is it possible to weld in a universal cat or does it have to be the whole exhaust manifold combine CAT?
Thank you in advance for the help

I have a2005 Mazda 3 hatchback with a 2.3L engine. It's firing this code at me so I'm wondering if it's possible for you to narrow down the possibilities. Last year it gave me the p0420 and I had one of the oxygen sensors and replaced the small a.c. Radiator which the mechanic said might clear it so I don't know which cleared it. If it is the cat, do I have to buy a Mazda specific one or will the sensors and computers accept non oem?

More than likely the problem is the catalytic converter. You don't have to buy an OEM catalytic, if available you use an aftermarket one.

That's what I was afraid of. Am I going to have to take it to a mechanic to check it our is there a way I can half ass test it without taking it all apart right now. My brother told me to listen to it to see if it rattles when someone accelerates gently on the throttle but I think it was a different problem he was referring to way back when. My light hasn't returned since I cleared it a couple days ago though.

i replaced the catalytic and the engine light came back on with the same code. What more should i do?

What model and year is your Mazda? did you use a new factory catalytic converter?

protege5 po 421. catalytic converter is new and everithing around sensor,pipes,spark plugs,coil,wires
and the code still

Did you use a new factory catalytic converter?

Car: 2005 Mazda 6s 4cyl
I have replaced the cat converter (exact fit -California emission) and 02 sensors (exact fit), but light came back on.. at a loss.. Any help appreciated.

Note: its the same p0421 code

Which catalytic converter did you replaced? Unfortunately, in some cases you may have to replace both catalytic converters.

I believe it was #2 (3-way).. My mechanic is as baffled as I am since he said I didn't need to replace the other because he found nothing wrong with it. He did throw out a suggestion of my gas cap which I haven't done.

Last year, sometime ago, I had someone run through to find the real problem, and he said it was the gasket. tbh I didn't think nothing of it after he threw out some over the top pricing, but maybe that could still be the problem?

I ran the code through a national database of repairs, and the most common problem for California for the P0421 code is the Warm-UP Three Way Catalytic Converter (#3) and no the Three Way Catalytic Converter (#2).
Also I have never seen a gas cap triggering the P0421, in any make or model, it's a totally different issue.

So my best bet would be replacing the other (#3) ?

Based on records from other shops and without me testing your vehicle personally, my best guess is that the Warm-UP Three Way Catalytic Converter (#3) has failed.

Where would my best bet be for buying one, besides Mazda? Im residing in California...
Thank you.

As far as I know there are no aftermarket available. Have your tried using high octane gas and fuel additives? Sometimes that will clean the cats

I have a 2014 Mazda cx5, I went to a Mazda dealer for the 96000km service, he propose the induction cleaning of the fuel system, the same day after the cleaning the engine light is on . I went back twice already they clear the p0421 and said it should not come back. After 50km of driving the light is back again. I'm planning to go back to see them any help would be appreciated.

Unfortunately they may have blown the Catalytic Converter during the fuel system cleaning service.

Thank you for your answer. I went back today, they agreed to change the catalytic converter, they didn't have it in stock, they said it will be there in a week.so, I will keep you posted.

Great!! Thanks for sharing. I'm glad they'll take care of the problem.

thank you for your reply, and apologies for not responding sooner.

I have not tried different gas or fuel additives. Where and what do you recommend I start with?

Again thank you!

Start by using high octane gas for about a week or two, if that doesn't help, then use the fuel additive. If the problem persist, then you may have to replace the Catalytic Converter.

I have a 2006 Mazda 6 with the P0421 error although i am not sure whether to buy the rear bank 1 or the front bank 1 catalytic converter

What is the engine size of your Mazda 6?

3L V6

Front Warm Up Three Way Catalytic Converter (RH)

Thank you very much!

I have something very similar, were they responsible for blowing out the CAT? did you get them to pay for it?

My wife has a 2017 Mazda 3 hatchback with 86,000 miles on it. We have already had to replace the engine at 64k, now at 86k the dealer says that it needs a new cat. Since we have replace the engine and now all of these things keep popping up, we are deciding to sell the car. This has been the WORST car we have ever owned, next to Infinity G35S.
If anyone is looking for a 2.0L engine with only 64k on it, the bearings, and entire inside of the engine is in perfect shape. The dealer conned my wife into a new engine with nothing wrong with it. So I kept it.

Mazda has gone down hill since Ford left.

Funny Greg-my local dealership tells me the parts of the engine build by Ford are terrible design and the new ones are all Mazda and much better......
Wife has a 2009 cx7 that's a mirror image of what you have. (well maintained vehicle too)
Its a 4 cyl 2.something; 2.4 I think . 76k new motor - under warranty - fortunately.
3k miles later, turbo (also under warranty) 3k miles later; wiring harness, after I take it home 3 days later; I am getting the P0421 and the dealer tells me its the CAT; Naturally, with the wiring harness; you can imagine my skepticism here. but everything seems to be ok with the connections, etc. you can't get them mixed up so I know they are in the right place on the harness.
I don't think the thing is plugged up; it seems to run good. What a pain in the butt getting it off for inspection; thinking about just replacing the sensors first to see if its a failed O2 before trying to pull the entire cat. I just wish the warranty would cover this one; but lucky for the MEP after more than 10k of dealer repairs:-) They are tired of seeing me....

I have 2012 Mazda 3 sport 114500 miles I got it at the dealer used car, after using it for 2 months the Check engine light up the code is P0421 warm up catalyst efficiency below threshold (bank 1) I replace the spark plug already , what are the other problem the causes it. Can you help me what are the thing to be checked. Thanks

usually caused by a sticking THERMOSTAT - you should notice that it's either not warming up all the way or that it takes way too long for it to warm up - start it up and let it idle - it should be to full temp in 5 minutes or less

I have a 07 mazda 6 4cyl with code p0421 which catylic converter i should change the one on top of bottom and do u have picture

Where do you live? California?

I live in hartford ct never been in california i brought the car out of massachusetts and it has a california emmission should i use regular epa and if so which one i need to replace..picture please

The one on top

So the exhaust manifold cat i have to replace?

Yes, on your vehicle the catalytic converter is part of the exhaust manifold

https://www.rockauto.com/en...

Ok thanks one more question?? Is it possible for me to change the cat without replacing the exhaust manifold as a whole unit ..and if so which one u prefer for my vehicle.pic please if u do have..thanks again

Unfortunately, you can't change just the cat.

Hi,

I have 2011 mazda cx 7 isv ...168 k miles

I got p0421engine light error .. so replaced both sensors but still getting same error.. getting frustration for putting money on this car...

Any suggestions??

I have changed both oxygen sensor but still getting same error..any suggestion

The most likely cause of the problem is the Catalytic Converter. If there are no exhaust leaks near the Catalytic Converter, replacing the Catalytic Converter will take care of the code.