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. 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.

 

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.

 

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.