P0421 code -- HELP!!!

I've had the P0421 (warm-up catalyst system below threshold) code pulled from my car twice. The first time the shop said that if it came on again, take it to the dealer due to the 8 yr emission warranty. The light came back on, I took it to the dealer, dealer pulled the same code & said that before Mazda would cover the replacement of the cat under the emissions warranty, BUT FIRST I had to have some work done to "clean" then engine since it was running "dirty", THEN if the light came back on, they would replace the cat. The work they want to do is (1) 4 cyl tune up - $50 (ok, maybe), (2) replace the plug wires - $136 (why? I have no problems with power loss or troubles starting it when it rains), (3) injector service - $90 (why? I only have 38,000 miles), (4) throttle place service - $80 (unlikely, but maybe). Obviously, I think they are milking me for $$, but I'm not exactly sure what I should do about it, or what the P0421 code is telling me is really wrong. Any suggestions would be greatly appreciated!!

Forgot to mention I have a '99 ES. Thx!!

Just had same problem Dealer replaced CATS with no questions Cause CAT Converter coming apart??
No cost car had near 71kand is a 98. The inpression I got was this may be common???

Has anyone else had this problem?

This is a quote from: http://www.obd-codes.com/trouble_codes/generic/p0421-warm-up-catalyst-efficiency-below-threshold-bank-1.php

A code P0421 may mean that one or more of the following has happened:

* The catalytic converter is no longer functioning properly
* An oxygen sensor is not reading (functioning) properly
* A sparkplug is fouled up
It doesn't sound like anything that they did is likely to help or was necessary (or correct) given the error code. I'd find a new shop.

Your car needs a CAT put on. The dealer was making sure that once they put the new one on it won't be killed by misfires / lean or rich condition.

Your car needs a CAT put on. The dealer was making sure that once they put the new one on it won't be killed by misfires / lean or rich condition.

Bump... I have the same issue in my 2001 ES, I let the spark plugs go too long, got some misfires and this code shows up. Took it to dealer and they said the pre-cat needs replacement, which sucks b/c it is almost $1000 :(

Is it advisable to get one off a junk car, or do those disappear due to precious metal content?
Are there any aftermarket options that are cheaper? Thnaks in advance

You can buy OEM @ http://www.catalyticconverterwarehouse.com , and have a local muffler shop do the install.

I have the same problem the error code kept coming on P0421 I finally got the cat converter and a new muffler. Less than 24 hours later the code came back so now what? My inspection sticker is due this month? Do I get the oxygen sensors fixed and if so which ones? Or should I get them both fixed ?

Doe's this car have Heated oxygen sensors (HO2), you might just have a blown fuse.

1) Visual inspect wiring to the H02 sesnors.
2) Roll the down stream H02 sensor from bank1 to bank2.

Downstream H02 sensor are the ones, after the catalytic converters.

OK, update.... I replaced the CAT, waiting to see it the code comes back. Got a Bosal CAT and some gaskets from PartsTrain for about $215. Of course they sent the wrong upper gasket... and as it turns out, only mazda dealers carry the upper gasket for like $28 (at least I couldn't find one). Also, the Mazda cat had studs to bolt to the front pipe... the Bosal cat had holes, so I had to buy exhuast bolts... another $8. Overall, it took me about 6 hours to pull it apart and then put it back together... and I am not terribly experienced, so someone else could prob do in half the time. I will post back if something goes wrong... like if the car catches fire etc... :)

Should have seen the old cat... it looked like a bomb went off... at least 2/3rds of the ceramic honeycomb was gone.

now the codes such as po420 or p420 they would replace no ? asked now 421 is a little different < now they wont replace the cat because your engine is running rich you wont notice all the time if ever but running rich and in some cases lean will kill a cat and then they would have to replace again< to much money for a warranty job, anything elts just ask or e-mail me anyone, and yes i am an auto tech and own a protege dx

Autoteck21 I saw your last post. I've got a Mazda 2002 Protege5 and am also getting the PO421 error code. Here's what I've had done in the last 3 weeks:

On 9/2/11 had following error codes: P0300, P1569, P0171
-replaced: cold air pipe, solinoid, water pump, timing belt, spark plugs, coolant.

9/4/11 car suddenly developed an idling problem at a stop light and check engine light came on flashing. As I started driving it barely had any power (I have a stick) so I drove it to a big parking lot where I knew I'd have it towed. As I pulled in it stalled on it's own. Mechanic said my fuel pump was bad and a miss fire code was present so had fuel pump it replaced. Misfire code removed.

9/7/11 picked up car day before, ran like a champ! In the afternoon I got in it and the idling issue was back and car had same issue where I could barely get any power and car is bucking, check engine light flashing again. This time I drive it all the way into the mechanic since I know it's a misfire. It wants to stall anytime I come to a slow speed/stop.

9/12/11 pick up car, mechanic replaced 1 coil pack free of charge, cleared out misfire code.

9/14/11 check engine light comes back on. I stop at autoparts store and get a misfire code P0300 and P0421.

9/19 take it to a different mechanic, they can't find anything wrong with it and the only code they get is the P0421.

9/27/11 stop at autoparts store to see if I'm getting any other codes besides the P0421, nope that's the only one.

I get the impression it is the cat. converter based on what I've read on here but have been unable to find an aftermarket part/s. Do you have any suggestions or ideas, or anything else to try in reference to my situation.

Many thanks in advance to anyone who can help!!!

Autoteck21 I saw your last post. I've got a Mazda 2002 Protege5 and am also getting the PO421 error code. Here's what I've had done in the last 3 weeks:

On 9/2/11 had following error codes: P0300, P1569, P0171
-replaced: cold air pipe, solinoid, water pump, timing belt, spark plugs, coolant.

9/4/11 car suddenly developed an idling problem at a stop light and check engine light came on flashing. As I started driving it barely had any power (I have a stick) so I drove it to a big parking lot where I knew I'd have it towed. As I pulled in it stalled on it's own. Mechanic said my fuel pump was bad and a miss fire code was present so had fuel pump it replaced. Misfire code removed.

9/7/11 picked up car day before, ran like a champ! In the afternoon I got in it and the idling issue was back and car had same issue where I could barely get any power and car is bucking, check engine light flashing again. This time I drive it all the way into the mechanic since I know it's a misfire. It wants to stall anytime I come to a slow speed/stop.

9/12/11 pick up car, mechanic replaced 1 coil pack free of charge, cleared out misfire code.

9/14/11 check engine light comes back on. I stop at autoparts store and get a misfire code P0300 and P0421.

9/19 take it to a different mechanic, they can't find anything wrong with it and the only code they get is the P0421.

9/27/11 stop at autoparts store to see if I'm getting any other codes besides the P0421, nope that's the only one.

I get the impression it is the cat. converter based on what I've read on here but have been unable to find an aftermarket part/s. Do you have any suggestions or ideas, or anything else to try in reference to my situation.

Many thanks in advance to anyone who can help!!!

If you are still misfiring then you should still get P0300 or something to that effect. When I had this issue (2001 Protege ES2.0, not a 5), it ended up being the coil packs. You mentioned you had one replaced... go ahead and replace the other too. P0421 is probably the Cat, I had to replace mine too, but it shouldn't cause the engine to misfire. If your car is setup like mine (and I think it is), there are two Cat's... the bad one is called the Pre-Cat and it is attached to the exhaust manifold. You can get an aftermarket Cat for about $300... if you get mechanic to do it, you could be looking at $800-$900 bucks. From other research I did on this board and others, the problem occurs like this: engine starts to misfire for some reason.... unburnt fuel gets dumped into the Cat which ruins it and the honeycomb falls apart... computer notices and throws that P0421 code.

Thanks for the response Chapper. I have replaced the 2nd coil since my last post. No more misfires. I'm having a difficult time "assuming" it's the cat. The car has never had the rotten egg smell, there has been no further hesitation, choking/puking. Also my mileage has remained the same. Thru further research I found a similar incident where the mechanic was ready to do the "standard" replacement of the cat and o2 sensor but decided to do some further DX tests and discovered it was the Mass Air Flow Sensor. I'm thinking it would be worth the $90 to take the car and have it hooked up to a diagnostic machine to rule out other possibilities. I've even wondered if it's a computer issue.

Any other feedback is welcome since it will be another week before I can get it to the shop.

Thanks for the response Chapper. I have replaced the 2nd coil since my last post. No more misfires. I'm having a difficult time "assuming" it's the cat. The car has never had the rotten egg smell, there has been no further hesitation, choking/puking. Also my mileage has remained the same. Thru further research I found a similar incident where the mechanic was ready to do the "standard" replacement of the cat and o2 sensor but decided to do some further DX tests and discovered it was the Mass Air Flow Sensor. I'm thinking it would be worth the $90 to take the car and have it hooked up to a diagnostic machine to rule out other possibilities. I've even wondered if it's a computer issue.

Any other feedback is welcome since it will be another week before I can get it to the shop.

I don't see how the MAF sensor, which goes to your intake, could be linked to P0421, which is an exhaust issue. Hesitation and puking... yes, the MAF is one thing in a long line of crap that can break. The point is, you have to make sure you aren't misfiring before you try to fix the P0421 issue... so if your engine is running fine, you can fix your exhaust issue.
Again, if your car has the same setup as mine, there are 2 Cats... the Pre Cat is right under the exhaust manifold, and the "normal" cat is further down the line in the exhaust (I think, this one will smell like eggs if it goes bad) For the precat, there are two O2 sensors, one before the precat and one after. This code gets thrown if the reading from the first O2 and the second O2 don't jive.
One thing I tried was to remove both O2 sensors and spray them with Carb cleaner to see if that helped. It didn't... the only thing left was replacing the Cat. You could get that diagnostic check done and see if they can tell you for sure if it is the cat or O2 sensors. Don't replace all of it if you don't have too... those O2's are pricey. Hope this helps :)

Note, I am not a mechanic, but i have the shop manual and I have experienced this issue and did the work myself to fix it... thus my comments :)

I don't see how the MAF sensor, which goes to your intake, could be linked to P0421, which is an exhaust issue. Hesitation and puking... yes, the MAF is one thing in a long line of crap that can break. The point is, you have to make sure you aren't misfiring before you try to fix the P0421 issue... so if your engine is running fine, you can fix your exhaust issue.
Again, if your car has the same setup as mine, there are 2 Cats... the Pre Cat is right under the exhaust manifold, and the "normal" cat is further down the line in the exhaust (I think, this one will smell like eggs if it goes bad) For the precat, there are two O2 sensors, one before the precat and one after. This code gets thrown if the reading from the first O2 and the second O2 don't jive.
One thing I tried was to remove both O2 sensors and spray them with Carb cleaner to see if that helped. It didn't... the only thing left was replacing the Cat. You could get that diagnostic check done and see if they can tell you for sure if it is the cat or O2 sensors. Don't replace all of it if you don't have too... those O2's are pricey. Hope this helps :)

Note, I am not a mechanic, but i have the shop manual and I have experienced this issue and did the work myself to fix it... thus my comments :)

That helps me. I didn't know where/which one the rotten egg smell would come from so that explains that. I'll try the carb cleaner trick too this coming week and let you know the results of everything. Thanks again for your helpful input!

Forgot to mention I have a '99 ES. Thx!!
http://www.mboxmusic.info/1.jpg
http://www.mboxmusic.info/2.jpg