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Old 01-02-2014, 08:26 PM   #1
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Default P0421 - Options...

Picked up a '99 last week with 75k miles. Ran great, but PO did say she throws a code intermittently and mentioned a "third" oxygen sensor. During test drive it never came on, then after purchasing and cruising around it came on and hasn't gone off. It also seems to have somewhat of a rough idle and sits around 900 rpm, but driving and going down the hwy runs fine.

Autozone printed a P0421. Took it to Mazda and without hesitation they told me I need a new cat and were wanting to charge me $457 (they didn't even hook it up). I know this code can be a number of things (02 sensors, etc) but wanted yall's opinion of possible options.

It is a CA emission car, so there is the precat and what a lot of y'all call a crappy header anyway. I did read some ppl convert to an '01 header.

Im thinking the cheapest route now is to buy a high flow cat and weld it in place of the old since it's not a bolt-on application. If I try to get an '01 header, what all entails to switch it over and get my CEL off?
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Old 01-02-2014, 08:37 PM   #2
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Megasquirt + Test Pipe?
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Old 01-02-2014, 09:22 PM   #3
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Quote:
Originally Posted by hornetball View Post
Megasquirt + Test Pipe?
^That's what I did with my '99 with the P0421. Very happy with my decision!
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Old 01-02-2014, 10:39 PM   #4
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Simulator.
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Old 01-05-2014, 02:22 PM   #5
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Quote:
Originally Posted by hornetball View Post
Megasquirt + Test Pipe?
Seems like an expensive fix for a CEL.

Anyone have experience with just swapping to a high-flow cat and clearing the light?
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Old 01-05-2014, 02:38 PM   #6
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A high-flow cat wouldn't solve the issue. There are two O2 sensors, the main one (to ensure proper AFR) and the downstream O2 sensor (to ensure the cat is working). The downstream O2 sensor should read less voltage than the main one, telling the ECU that the cat is functioning. When I used an OBDII scanner, I saw that the downstream O2 sensor read 0 volts all the time, meaning it was likely dead. I never did replace it to check if that was for sure the issue, though. But, if you get the code with the stock cat, you'll get it with a high-flow one.

Yes, Megasquirt is an expensive fix for a CEL, it's more so the best thing you can do for the car, and lack of CEL's is an added benefit
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Old 01-05-2014, 03:32 PM   #7
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I guess I want to know the cheapest route. Should I convert to fed-spec? As in, replace mine with a '99 header and maybe put in a new rear cat in case it's already fudged up?
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Old 01-05-2014, 03:35 PM   #8
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This is the thread you want: New owner got burned - Miata Forumz - Mazda Miata Chat Forums

Quote:
Can you get away with ditching the pre-cat manifold and still be legal in AZ?

I also have a 2000 CA emissions spec Miata, but I'm currently running a 01-05 NB factory exhaust manifold (with heatshields) and 01-05 EGR tube. It bolts right in and works just fine with the stock ECU. The only catch is that you have to move around the O2 sensors. I'm not saying its 100% legal, but I did pass both CA visual and sniffer emissions on just the cat in the midpipe... twice.

My motivation was 4 fold:
Save weight: The 01+ parts weight a fraction of the 99/00 iron boat anchor
Performance: The iron is a double killer. Not only is it heavy, it flows like crap
Cost Savings: $800 for the CA-EPA approved replacement pre-cat is not cool.
Cost vs Performance: At the time I bought all the 01+ parts to do the swap, it was $75 + shipping. Sadly, the cost has gone up.

While the 01+ OEM parts are no Racing Beat long tube, it does flow quite well. It also has the benefit of a heat shield and OEM quality fit and finish. My 99/00 CA emissions manifold/cat/egr tube are living quite happily under my workbench should I ever need to call them back into service.

The entire swap with pictures can be found here: http://peatpics.com/gallery2/main.php?g2_itemId=29359
The peatpics site seems to be iffy recently, but to give you a better idea of what I'm talking about, you get to replace the cast iron manifold with this:



Hows that for better flow?

After you replace the hardware, the process for moving the sensors is as follows:
  1. Get the Racing Beat "NB header extension" o2 wire extensions and move the CA "rear" o2 sensor from the downpipe to behind the cat in the midpipe. This will require welding a bung in the midpipe. Clearance can be an issue.
  2. Install the "front" o2 sensor in the midpipe, where the CA model had the "rear" one installed.
  3. Win.

This of course assumes your main cat in the midpipe is good. 90% of the time its fine, the front cat does all the work and the rear one is "along for the ride."
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Old 01-05-2014, 03:43 PM   #9
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Awesome! Thanks for all of that ^^^^

And yea, peatpics isn't loading...
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Old 01-05-2014, 04:08 PM   #10
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To make this easy on you, here is a parts list:
  1. 01/05 Manifold (used)
  2. 01/05 EGR Tube (used)
  3. o2 Sensor Wiring Extensions: Race Header Kit for 99-00 Miata - Racing Beat
You can reuse the CA spec midpipe and current o2 sensors without issue, assuming they are still good.

You may be able to get away with only the 51" extension, but I found my front wires to be too short and had to fab up an extension on the fly so I recommend just getting the set listed above.

You may also need:

NUT,FLANGE (JE10-40-355) - $3.38 - JE1040355 x3

STUD (2306-13-456) - $5.39 - 230613456 x3

GASKET,CONVERTER (BPM1-40-450A) - $13.75 - BPM140450A x1

GASKET,EX.MANIFOLD (BPD3-13-460) - $13.96 - BPD313460 x1

New gaskets are always nice, but not always required. Every time I've had to remove the manifold I've had the studs and nuts get buggered up so those should be the replacements. YMMV
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Old 01-05-2014, 04:17 PM   #11
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Also, for the love of $DEITY get yourself a can of PB Blaster & and "wobble" extension to tackle the triangle flange between the precat and downpipe. Start soaking it like 3 days in advance and learn to love the smell of burning PB
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Old 01-05-2014, 05:02 PM   #12
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Quote:
Originally Posted by EO2K View Post
Also, for the love of $DEITY get yourself a can of PB Blaster & and "wobble" extension to tackle the triangle flange between the precat and downpipe. Start soaking it like 3 days in advance and learn to love the smell of burning PB
Guess I'll also need an o2 bung?

**edit - nvm, looks like the harness kit supplies one. Thanks
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Old 01-05-2014, 05:08 PM   #13
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Yep, you will need it for the rear sensor after you move it to the midpipe, after the main cat. There is one included in the Racing Beat wiring extension kit I listed above (looks like a thin hex nut) but you could always source a bung from elsewhere as well
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Old 01-05-2014, 05:11 PM   #14
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As far as the rough-ish idle I mentioned above, could this '01 header swap (cleared CEL) help with this? Like I said, it idles around 900 rpm when warm and is kind of rough. While mobile it's smooth.
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Old 01-05-2014, 05:14 PM   #15
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No. You have other problems with the car as well. Instead of thinking there's some magical fix for all this, you should actually diagnose and fix the problems.
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Old 01-05-2014, 05:15 PM   #16
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No. You have other problems with the car as well. Instead of thinking there's some magical fix for all this, you should actually diagnose and fix the problems.
What the hell do you think this whole thread is for? My dad is a mechanic and said a bad o2 sensor could effect idle. Refer to first post. Wanted y'all's opinion as well.
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Old 01-05-2014, 05:18 PM   #17
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Quote:
Originally Posted by Dustin View Post
What the hell do you think this whole thread is for? My dad is a mechanic and said a bad o2 sensor could effect idle. Refer to first post. Wanted y'all's opinion as well.
THE ONLY OXYGEN SENSOR THAT AFFECTS YOUR CAR'S OPERATION IS THE PRIMARY ONE!!
You have a faulty secondary, and/or bad cat.

This thread is for spoon feeding you info taht you coulda searched and easily found.

your dad is a crappy mechanic if he thinks a secondary o2 sensor does anything aside from check to make sure a cat is there and working.

Refer to this post.
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Old 01-05-2014, 05:27 PM   #18
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Quote:
Originally Posted by 18psi View Post
THE ONLY OXYGEN SENSOR THAT AFFECTS YOUR CAR'S OPERATION IS THE PRIMARY ONE!!
You have a faulty secondary, and/or bad cat.

This thread is for spoon feeding you info taht you coulda searched and easily found.

your dad is a crappy mechanic if he thinks a secondary o2 sensor does anything aside from check to make sure a cat is there and working.

Refer to this post.
Refer to previous post - never mentioned to him it was a secondary o2 sensor, simply stated "o2 sensor." This was my bad when communicating with him.

I'm still learning and haven't been trolling this forum for however many years you have while living in your mom's basement. Seems she would'a taught ya, "if ya ain't got nothing nice to say, don't say anything at all."

Every thread I read that you post in is rarely ever any good advice, but sounds as if you're responding to respond. Now, E02k on the other hand, offered up some great advice that I can now solve my CEL problem with.

Thanks for nothing. Again.
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Old 01-05-2014, 05:34 PM   #19
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Quote:
Originally Posted by Dustin View Post
Refer to previous post - never mentioned to him it was a secondary o2 sensor, simply stated "o2 sensor." This was my bad when communicating with him.

I'm still learning and haven't been trolling this forum for however many years you have while living in your mom's basement. Seems she would'a taught ya, "if ya ain't got nothing nice to say, don't say anything at all."

Every thread I read that you post in is rarely ever any good advice, but sounds as if you're responding to respond. Now, E02k on the other hand, offered up some great advice that I can now solve my CEL problem with.

Thanks for nothing. Again.
I don't care what you have or haven't mentioned to your crappy mechanic dad.
The info I gave you is on point, and I'm not gonna stroke your **** and spoon feed you like EO2k, when your follow up question is clearly unrelated to his advice in the first place.

You're a classic example of an ungrateful POS n00b that wants his hand held and butt cupped every time you ask a question.

I'll make sure to never give you any good advice since you can't handle it, you're welcome.
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Old 01-05-2014, 07:47 PM   #20
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Quote:
Originally Posted by 18psi View Post
THE ONLY OXYGEN SENSOR THAT AFFECTS YOUR CAR'S OPERATION IS THE PRIMARY ONE!!

your dad is a crappy mechanic if he thinks a secondary o2 sensor does anything aside from check to make sure a cat is there and working.
This is true, on our cars specifically. Yes, OP should've searched and found out that information on his own. But, it is common that the downstream O2 sensor is used to fine tune the outputs, so it's not totally ridiculous to think that the downstream O2 affects the vehicle.
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