NB Cat Delete CEL fix question
 

I know most of you guys are pretty significantly modded, but hopefully someone has addressed this... My daily is a stock 99 miata w/ racing beat exhaust. The exhaust does not have a cat, but it has a secondary o2 sensor tapped in it. The sensor is extended from the exhaust a bit, apparently in an attempt to make it not throw a code. Unfortunately it still throws a code every couple hundred miles or so. If I just pull the sensor and tie it up somewhere under the car, then plug the whole in the exhaust, will I still throw a code? Any other options I could consider other than the "black tape over the CEL" method? Thanks.

They make a O2 simulator that simulates a working O2 sensor. cjernigan runs one on his 99' with success IIRC. Ask him what brand it is. And no, you can't just remove it and expect the CEL to go away.

FWIW, my CEL stays on all the time. I don't care. Doesn't bother me.

A simulator is illegal, so whatever I write in this post is purely hypothetical. This fix may or may not be working great in my car. ;)

Car had a CEL for "emissions too close to threshold" or something like that after a few months with a high-flow metalcore cat.

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 '04 at least) are: 1., black with white stripe (power), 2., white (signal in), 3., red (mostly wrapped in a second layer of white insulation - signal out), 4., black with red stripe (sensor ground).

Cut the red and black/red wires. 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. 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.

Done.

This sends an acceptable voltage to the ECU, so it thinks all is well. (These aren't the droids we're looking for .... move along .....)

Unplugged, or connected outside the pipe?

the 2nd o2 sensor in every vw, honda, subaru, or yugo I've ever worked on was just a check sensor to make sure the cat works...so you can hang it outside the exhaust pipe and it won't throw codes.

Ahh, thanks for the info. I'll have to look into the simulator, that stupid light drives me crazy. I always have to reset the ecu right after it comes on. :crx: I have figured out that if you pull the battery off for right around 20 seconds it resets the ecu, but leaves the radio presets and trip odometer.

edit: Thanks for the writeup evank! That's a lifesaver.

excellent writeup....

Ahh, so maybe my plan will work. I left the sensor connected outside the pipe so it's sniffing clean fresh air. I already tried unplugging it and figured out that didn't work. :giggle:

I dont think that is such a good idea. Not sure if miata's are the same, but on a subaru it throws a code for too low of a reading also.

I cranked it up and it's not throwing a code immediately. Hopefully it's good as it is. If it throws a code I'll go with the resistor. Thanks again for the help everyone. :bigtu:

You can also try the the anti-fouler mod.

Get a pack of two spark plug anti-foulers from autozone, drill them out to accept the o2, stack em, install!

worked on other cars other than miata, i had hydra on my 03, so dunno if it'll work.

It will eventually. I was doing the same thing back when I was using the emu and it always picked it up eventually just at really random intervals.

Yeah - I think I still have mine laying around somewhere - but it wasn't just a cap - I think the OBD-II miatas are more picky, you need both a resistor for the "heater" and to have a time delayed signal. I bought a little box, which might be for sale..

I tried that but it didn't work, so I went with the electronic version.

I read on a Subie forum that sometimes it takes two of the anti-foulers. There was only room for one in my car.

I've used anti foulers on my rsx's and all it does is prolong the time it takes the ecu to figure out you dont have a cat. it is an ok fix, but every now and then you will STILL get the cel. Dont know how it is on subaru's since I just turned off the cel with the opensource tuning software on each one. Pretty much your choices are:
1) do the resistor mod
2) ignore the cel
3) put in a good cat

I do have a good cat. TurboTim built my exhaust with a metalcore high-flow cat. A few months after that is when I started getting the CEL.

I'm sure you have a good cat, but is it the stock mazda cat? I'm sure its not, probably a hi-flow cat that the mazda sensor/ecu doesnt like:)

You guys saying it would still throw a code were right. I made it 10 miles and it came on again. I've run the anti-foulers on my old s2000 and never had a problem. The exhaust I have is already extended enough that I don't have room for the anti-fouler and that's not enough. I'm going to try to the resister mod when I get some time.

I misunderstood your definition of "good". :)

I'd be very curious how that works, with the resistor and the cap - I'd heard that the CPU looks for a significant delay and sends little "test signals" in the form of wavering AFR's... But if it works it's a great trick and should be stickied, beats buying a $80 box!

That there's yer problem.

Git you one a them fancy simulators fer forty bucks and be done wid it.

Universal o2 simulator single output [unv-o2sim1] - $40.00 : O2 Simulator, E-commerce

Problem solved. And solved correctly.
http://www.o2simulator.com/images/la...sal_full_S.jpg

Leave o2 sensor in place. Snip signal out wire and replace signal with analog output from appropriately programmed LC1. Done.

I'm dong this, but a bit more complicated as I'll have the LC1 "move" switchpoint up one point from stoich to 15.7:1. Not a problem to fool the computer on both sensor since LC1 has 2 analog outputs.

obd1 > *

My 99 had 2 cats from the factory. I cut off the 2nd cat, and used a piece of pipe to replace it. The 2nd O2 sensor is after the 1st cat. When I went turbo, I replaced my stock manifold and DP with aftermarket ones, which now I am catless. This made my CEL come on. I bought a simulator. Then I went to the local u-pull-it and snipped the pigtail off an O2 sensor that fit my car. The I wired the simulator directly to the pigtail and plugged it in to the secondary O2 harness on my car. I hid it so Mr. Inspection can't see it. I don't have a CEL anymore.

I was totally happy with my simulator. I used a resistor instead of the sensor to save a bit of weight, and then all the wiring was under the dash instead of running it out under the car. Can't say I minded it, though my standalone also doesn't turn on the CEL, and I have a used O2sim for sale. :-)

Damn, man. I just ordered one Friday.

Hi MT folks -- sorry to bring back an old thread. Recently a bunch of people contacted me asking for a diagram. So I am complying. Here's an ugly diagram, but it's accurate.

http://www.snarc.net/o2diagram.jpg

What about pulling out the CEL bulb?

when you say forward of the resistor. is forward towards the o2 or towards the ecu.


also i have a 63v microfarad capacitor laying around, will that do the job? it's also polarized so witch wire would go into the + side?




edit: didn't see page 2 with a sweet ms paint how to.

I don't know.

I am going to do this CEL mod on my 04 MSM (no cat) but had a question about whether I should use a polarized or non-polarized capacitor. Also, the capacitors range from 50v to 350v so I wasn't sure which one to use. I am hoping that based on the picture below, someone can tell me which one they successfully used to complete this mod. Thanks for your help!
https://www.miataturbo.net/attachmen...ine=1324700653

Shouldn't really matter. Most electrolytics are polarized, the non-polarized caps are typically used in applications where they're exposed to AC waveforms, such as in speaker crossovers. In this application, a polarized cap is fine as one leg is going to ground.

That rating indicates the maximum voltage that the cap can be exposed to without failing. Since the maximum voltage generated by an O2 sensor is a little under 1 volt, any cap you can find will work just fine.

I appreciate your response. I'm going to go ahead and use the polarized 160v capacitor.

What voltage does the ECU want to see from the post-cat O2 sensor signal line to make it happy? (The cat I have is somewhat "less than optimal" from the ECU's point of view as compared to the original OEM cat + the post-cat O2 sensor has been abused to death...or maybe the wrong one, I don't know but I'm tired of f'cking around with it - it is inspection time again dammit).

sorry for bringing up a dead thread...but after reading this, i had a question.

after putting that cap between the red/black and red wire, do you just leave the part of the red/black wire that goes to the sensor itself unhooked? or do you re-solder it to the node/joint where the cap is already soldered to the red/black wire?

There are mini-cats now that go between the bung and the O2 sensor. Much easier than fooling the ECU or jacking with the wires ... the O2 sensor sees exhaust air that is post-cat, like it's supposed to. The main cat can then be deleted if so desired.


Edit: Mini cat from Big Daddies Garage.

Dynotronics makes one as well; appears to be of higher quality but significantly more expensive.