Check Engine (PO421) trouble.
 

hi there,
i have a new problem with my car. dont think its to serious this time though. my check engine is back on and the OBDII code reads PO421 which is: warm up catalyst efficiency below threshold (bank 1). not to sure what is it but my friends and I are thinking it might be the catalyst converter sensor. has anyone had this problem yet or know what it is? please help. thanks

If you haven't done any modifications, it could be two things.

1) Bad second O2 sensors (most likely).
2) Bad cat

Both should be covered under warranty if the vehicle still qualifies. I think emissions stuff might be warranty'd longer, but I'm not sure.

yeah, thats what I was thinking. major performance mods on my car injen downpipe and an OBX muffler. had those 2 parts for over a year now, so im guessing its a bad sensor somewhere. the thing is that my car isint under warranty anymore for a long time now, but ill try to figure it out. Thanks though.

ok, here i am back to this same problem. since i last posted this thread i've replaced both o2 sesors which cost lots of money and i even checked out the exhaust manifold for any cracks but dont see nothing. I've reseted the codes many times already and the CEL keeps coming back. so, now one of the only thing left is the catalytic converter. I have 93,000 miles on my car so if i think on replacing it, would that fix my problem? if maybe or so, where can i get a good one? any suggestions will help. thanks

the precat after the exhaust manifold is becoming common to replace. the only direct fit replacement for that is from the mazda dealer. i have several in stock if interested.

hey there,
yeah, i would be interested to finally be able to fix my problem. let me know how much it would be. if the price is reasonable i wouldnt mind getting it. thanks

Or you could just buy a header and to hell with OEM.

yeah, i was thinking about that but im worried that if i remove the pre cat, i dont think its street legal, such as passing smog since a catalytic converter is gone. also, would that cause the CEL to go on, although mine is on right now, thats the reason why i wanted to change the pre cat in the first place. i sent an e-mail to protege5online.com's sale tech about the AWR headers and they told me they are not street legal. but no one as gotten back to him saying anything bad about it whether they passed smog or not. but some smog shops wont know if there is a pre cat missing since some other cars dont have any. but of anyone out there as more info, please let me know, thanks.

The only 'legal' header ever made for the Protege was the Wagner shorty, but not only are they out of business the header didn't eliminate the pre-cat, which is your problem anyway.

Any removal of a catalytic converter is technically illegal. As for emissions testing though it depends on your state's laws.

BTW the AWR is a nice piece but very pricey. Also, protege5online...you may wanna read up on them before you order.

i know this is very old but did you ever fix the problem

He had a clogged precat. It's the cat that comes right after the stock exhaust manifold.

FWIW, I had a Wagner shorty on my P5 for awhile. It literally fell apart. No wonder those guys went out of business.

I put a header on mine and the check engineblight. Came on so I added an o2 spacer and it went of. Cost ten bucks off eBay.

Hi. U put the spacer on both o2 sensors?