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Discussion Starter #1 (Edited)
A simulator is illegal, so whatever I write in this post is purely hypothetical. This fix may or may not be working great in a friends car. Car had a CEL for P0420 after a few months with a high-flow metalcore cat. FM Cat Midpipe package.
Under the driver's seat, very close to the trans tunnel, is a plastic-wrapped series of four wires. This is where the wires exit the car and go underneath to the rear O2 sensor.
Cut open the plastic wrap. The four wires (on an ‘99 at least) are: 1. black with white stripe (power), 2. red with yellow stripe (signal in), 3. red (mostly wrapped in a second wire- signal out), 4. black with red stripe (sensor ground).
Cut the red and black/red wires. The wire surrounding the red wire, cut it shorter and just isolate it with tape. Install a 1 megaohm resistor (1/4-watt or 1/2 watt) in the red wire. Then install a 1 microfarad capacitor jumping the red and black/red wires, forward of the resistor(nearest to O2 sensor). Cover the wires with electrical tape or (better yet) heat-shrink tubing. I used electrical tape but I'm told it could melt over time.


'99 Miata schematic
 

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Discussion Starter #5
"Very nice. Is there a way to do this so that we can remove the O2 sensor completely (ie. cutting it out of the harness)?"

I think you would have to find out the right combination of resistor and capacitor. Or this combination works alone by it self with out the connection of the O2 sensor, this I dont know. My friend would have to, hypothetically... try it.
 

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Discussion Starter #6 (Edited)
"what is this for?"

Many people purchase high flow cats that dont change the signal of the second O2 (rear) sensor well enough over a short period of time. They get CELs well before the manufactures suggest their cat will fail. My friend had his CEL go on within 3 months of his purchase of an aftermarket cat. This seems to be hypothetically working.
So far I am aware of this mod may work for the '99-'00 miata. The '01-'05, have different color wires but the principle remains the same.
 

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why dont you put a non fouler on the oxygen sensor and drill the other non fouler out the diameter of the oxygen sensor so it will fit correctly and either weld it in or use the original port? I did that with my protege and it worked without a cel. you should give it a try


This will cure the Cel caused by removing the Catalytic converter, usually when you install headers.

Total time required: 15-20 min.

Tools and Parts required

Tools:
Adjustable wrench & vise grips or another adjustable wrench (you need 2)
Drill with 1/2 inch drill bit
Clamp if availible, (Vice Grips and a foot will work too, if you have no clamp)

Parts required
2 Spark Plug Non-fouler adapters 18mm w/gasket (Napa Part 7302442)
2 come in a pack.



Step #1
Disconect the negative terminal on the battery, then Take ONE & only one of your 2 non-foulers and Drill out the Top end with the 1/2 inch drill bit untill it goes all the way through.

You can eithor clamp the peice down to be drilled by hand, or do what I did & clamp some vice grips onto it, hold it down with your foot & drill away. You can also use a Drill press if you have access to one, but its easy to do it with a hand held drill.

Step #2
Attach it to the un-drilled peice making sure the Un-drilled peice screws On Top of the drilled peice & making sure there is a gasket on.
Step #3
Unclip the o2 sensor clip & Disconect the o2 sensor (2nd o2 sensor for removal of Cat 1 with headers install) from the car using the adjustable wrench.

IMPORTAND to unclip the o2 sensor first so that you can spin the wires while you unscrew it to insure you dont over twist the wires.

Step #4
Screw the o2 sensor into the non-foulers making sure you have a gasket on using eithor 2 adjustable wrenches or 1 adj wrench and vice grips. It should look like so...
Step #5
Reconect the o2 sensor to the car and plug it back in. It should look like so...
 

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Discussion Starter #8
Nice. But not as stealthy.

Update: My friend still has not gotten a CEL. Checked OBD with scanner and nothing showed up. Looks like a winner.
 

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I didn't see any Capacitors that said microfarad In radio shack is there another name for this or where's a good place to purchase one online?
 

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"Hypotheticaly" speaking, just remove the rear o2 sensor I
& let it sniff ambient air, guess what, no code!
 

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I believe the sensor has to detect something other than fresh air or the ECU logic will assume the sensor connection is open. Try it and let us know.
 

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I didn't see any Capacitors that said microfarad In radio shack is there another name for this or where's a good place to purchase one online?
It will probably be labeled as μF (mu+F = micro Farad). Check for THIS or THIS.
 

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I have a 2002 base model. Manga flow exhaust. My check engine code reads catalytic converter fail. It was replaced last year. Any suggestions?
 

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why dont you put a non fouler on the oxygen sensor and drill the other non fouler out the diameter of the oxygen sensor so it will fit correctly and either weld it in or use the original port? I did that with my protege and it worked without a cel. you should give it a try
I did this on my previous '99 Miata. It didn't work but the converter was the original with 75k miles and I could smell that it was bad.

I bought it from an unscrupulous seller that lied to me about the dead battery, dark current drain from his faulty dimming rear view mirror install that kept killing the battery, and a dead catalyst. When a low mileage (40k) 2001 LS popped up in nearly flawless shape, I sold the '99 at a loss to a guy that wanted a base model to race.

I'm not saying the plug fouler option isn't worth a try but in my experience, the '99-'00 Miata ECU is too sensitive and it is doubtful this will work.
 

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So will this run a pre cat monitor assuming you resistor the bank 1 upstream O2 sensor?
This thread turns 5 years old in a little more than a week. It is ridiculous to expect OP to reply to you.
 
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