Check engine light code PO421

[akazempoor]

Hey Guys,

I am new to the Miata game sorry. I have a 1999 mazda miata that I got 3 weeks ago. I was driving one day from work and I got a check engine light. So I did a oil change, changed my spark plugs, and also checked the gas gap. I reset the check engine light by disconnect and reconnect the battery. My check engine light came on again couple days ago. I went to Pep boys and they read it for me and it said Code PO421. What can I do to fix this? I heard my car is a CA-SPEC car. Please help

[mini_fd]

I believe it's the 02 sensor. The top one by the cat in the header.

Here is a link to answer your questions...

http://www.google.com/search?q=p0421miata+site:forum.miata.net

[kung fu jesus]

Today is your lucky day! :)

No, really though, I had the same issue with my '99 CA emissions Miata, I would clear the code out with an OBD2 scanner tool and it would come back every so often.

There are a few issues that cause this:

Failing pre-cat (let's call it "PC"), failing oxygen sensor (O2 sensor - the front one, CA emissions cars have two), a combination of the two, and it could also be a failing coil pack which is choking up the PC.

The first time I had this issue, I replaced the PC. It is expensive, difficult to get to (you will have to remove the exhaust manifold, EGR tube, front O2 sensor and down pipe (DP). If you are in CA, companies will only sell you a CA-legal cat to bolt in. This adds more cost. So, I bought a new PC, new front O2 sensor and cleared the code. Less than 2 years later the code came back. Grrrr!

I am sure if you search "Miata, P04321" a LOT of information, threads, and fixes will come up. Some people use an O2 modifier, which is a simple metal collar that you screw into the bung where the front O2 sensor goes, then you screw your O2 sensor into that. Essentially what it does is it pulls the sensor out of the exhaust flow a little bit so it can't 'taste' the bad emissions. It's a cheap fix. It is really only a temporary fix, though. At some point, if you live in an emissions-testing state, a tailpipe test will come back bad and you will fail, despite no CEL (check engine light).

There is another option...

This is a performance option that works great, allows you to pass smog, visual, AND get some really nice power by uncorking the exhaust on your car. I did this and it not only cured the CEL, but gave me a really nice power bump!

You can remove your restrictive OEM manifold (header) and PC and retrofit it with a non-CA emissions OEM unit. Here is what you need...

1) 99-00 non CA emissions OEM exhaust manifold (header), or any '01+ OEM header and '01+ EGR tube (important, only needed if you use an '01+ header)
2) the OEM heatshield from the above (to pass visual inspection easier...everything looks factory)
3 a) an exhaust shop to cut and weld a new O2 sensor bung into your existing midpipe.
3 b) a new 2.25" midpipe with an O2 sensor bung welded into it.
4) exhaust gasket between the downpipe and the midpipe
5) some shielded wire to extend the wiring on your rear O2 sensor
6) *maybe* some mounting hardware to connect the header to the downpipe.

Altogether, this cost me about $450. A new PC, will run you about $250-500. You can see what I did in the build thread here on page 1 ( http://mazdaroadster.net/showthread.php?5638-Blue-Lion-Resurrection&highlight=blue+lion)

I am trying to find the site I used that documented someone else's process. If you need it, let me know I will try to find it.

The method above is specifically for making everything look kosher for strict inspection, as done in California. If you DON'T live in CA, you can easily replace parts 1 and 2 with an aftermarket header.

The '99-00 non-CA OE header is a cast iron unit. It will flow better than your exitisting setup, but the '01 + header is a 4 to 1 tubular unit that flows even better still and puts you within 3-5 hp of a more expensive aftermarket header. You can get parts 1 and 2 from our forum sponsor Project G. Call them, they are great people!

The key issue most people over look here, despite the solution you use to clear the code, is something is coking up the exhaust. The '99-00 Miatas are known to eat up their coils. A weak ignition pumps more bad emissions through the exhaust, fouling the cats and sensors, so I would strongly recommend replacing the coil pack. I did when I did the manifold retrofit and it was night & day better at idle and everywhere in the power band. It was a subtle improvement, but noticeable. I found a NEW coil pack on amazon here:

http://www.amazon.com/gp/product/B00AZMI9EM/ref=oh_details_o00_s00_i00?ie=UTF8&psc=1

I did the homework on this coilpack. It is a direct OEM replacement. You can read my review on it there. It is a quality part and I see the price on it has actually gone down a little since I bought mine. $80 for a new coilpack, already mounted in the bracket, this is an ABSOLUTE_STEAL. Buy it now. No, really, buy_one_now. Online, 'Mazda' OE coilpacks go for $200, unknown used ones go for $80.

If you need more explantion or tips, let me know by replying to this thread. If you are in CA, going with the OEM look is crucial.

[kung fu jesus]

The 99-00 CA emissions cars have a different ECU, and slightly different code numbers. The emissions system has two cats and two O2 sensors. The PC on the CA cars reaaaaaaly chokes the power. The cast iron manifolds on all the 99-00 cars is heavy and doesn't flow as well as the '01+. It's a shame.

The '01+ non USDM OE manifold (Europe, Asian markets) is really something quite nice and *looks* aftermarket, but is hard to get and cost prohibitive because of shipping.

The key to your issue is this: your ECU is getting an errant reading that something is out of spec. You can replace the parts because they lose efficiency from age and use, but it really doesn't always cure the problem. This is why my PC went bad after a short period. You have to think about what makes a car push out more particulates (emissions), and that usually stems from unburnt fuel caused by a bad ignition. We all know plug wires are a common issue, but the coils need some attention too. As mentioned above, the '99-00 coils are a known weak spot, so as they degrade over time, you may not really notice it. Those $80 coils on Amazon I linked are great insurance that no matter what fix you choose to cure the p0421 code, you won't be destroying it like I did in a relatively short time. :)

[akazempoor]

The 99-00 CA emissions cars have a different ECU, and slightly different code numbers. The emissions system has two cats and two O2 sensors. The PC on the CA cars reaaaaaaly chokes the power. The cast iron manifolds on all the 99-00 cars is heavy and doesn't flow as well as the '01+. It's a shame.

The '01+ non USDM OE manifold (Europe, Asian markets) is really something quite nice and *looks* aftermarket, but is hard to get and cost prohibitive because of shipping.

The key to your issue is this: your ECU is getting an errant reading that something is out of spec. You can replace the parts because they lose efficiency from age and use, but it really doesn't always cure the problem. This is why my PC went bad after a short period. You have to think about what makes a car push out more particulates (emissions), and that usually stems from unburnt fuel caused by a bad ignition. We all know plug wires are a common issue, but the coils need some attention too. As mentioned above, the '99-00 coils are a known weak spot, so as they degrade over time, you may not really notice it. Those $80 coils on Amazon I linked are great insurance that no matter what fix you choose to cure the p0421 code, you won't be destroying it like I did in a relatively short time. :)

Man thanks for the awesome response. I sent you a private message.

[LoveJoy33]

I have the same code popping up in my car. The pre-cat is dead in the downpipe. I think the only fix for that is a header really....

[kung fu jesus]

The pre cat is replaceable, but it's a bear of a job. It's a lousy design, but there are work arounds. :)

[mcadventures]

I'm just curious if you still have this car. I have a 99 10ae with the same issues and I'm trying to decide if it's worth putting anymore money in to it. I just had a new clutch installed, and a rebuilt 1.8 engine installed. And now this issue is back (had it previously but replaced the sensor and was okay for a while). I realize this is an old post and may not be "seen" but just posting it out there...TIA.

[kung fu jesus]

I don't have the car anymore.

If you live in CA (I can't see your location while posting from my phone), you can *try* the retrofit, but I make no guarantee it will pass visual inspection. I think the key in that case is to make it all look as stock as possible, including OE heat shields.

If you don't live in CA, I would try the retrofit and O2 relocations. An aftermarket header would be an option too, again with O2 relocations.

[miatadave22]

The 99-00 CA emissions cars have a different ECU, and slightly different code numbers. The emissions system has two cats and two O2 sensors. The PC on the CA cars reaaaaaaly chokes the power. The cast iron manifolds on all the 99-00 cars is heavy and doesn't flow as well as the '01+. It's a shame.

The '01+ non USDM OE manifold (Europe, Asian markets) is really something quite nice and *looks* aftermarket, but is hard to get and cost prohibitive because of shipping.

The key to your issue is this: your ECU is getting an errant reading that something is out of spec. You can replace the parts because they lose efficiency from age and use, but it really doesn't always cure the problem. This is why my PC went bad after a short period. You have to think about what makes a car push out more particulates (emissions), and that usually stems from unburnt fuel caused by a bad ignition. We all know plug wires are a common issue, but the coils need some attention too. As mentioned above, the '99-00 coils are a known weak spot, so as they degrade over time, you may not really notice it. Those $80 coils on Amazon I linked are great insurance that no matter what fix you choose to cure the p0421 code, you won't be destroying it like I did in a relatively short time. :)If in doubt replace the cam angle sensor. I chased this issue for months. I'm old and don't remember the code, P0421? They run from $35-75. 15 minutes to change and walah, codes and problems gone. Be sure to get the new style, it is less heat sensitive. If you have over a 100k on your car, replace it. Consider this a maintenance item.https://uploads.tapatalk-cdn.com/20171013/44d7064e715a5ee03c2aa44baa50c344.jpghttps://uploads.tapatalk-cdn.com/20171013/979ae1139b3b15f0c3e26d36f9f89567.jpg

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[kung fu jesus]

Nice! The one on our 10ae flaked when my wife drove through some deep standing water.