When I had my Civic the guy passed it over Camera they have in the ground for looking at cats. The guy proceeded to tell me I had no cat and I was going to fail. I had to force him to look under the hood to see where it actually was.

I've heard the same. I'm talking out of my ass here, but it could have to do with the newer EGR and other emission systems that draw off the exhaust to go back into the intake, or something like that. Lower back pressure means disruption to the flow to the intake, something like that.

Also, removing the CAT on a new car will mean invalid readings from some of the sensors the engine uses for it's trim levels, so it could make the ECU run the engine at less than optimal settings.

I had my 99 4runner for a while with no issues. Recently I drove down from the US to Belize. As soon as I ran through my first full tank of gas here I pulled a P0420. I have read that poor gas quality can contribute to the code popping. Would this mod take care of the CEL as well as the poor MPG that I am getting?

An internal combustion engine needs a catalytic convertor like your butt hole needs a little plastic tube with a dryer sheet crammed in it...Sure it makes the exhaust a little less offensive to others and to the environment, but your butt hole doesn't really care at all...BBRRRRTTTT

It's only the exhaust sniffing fuel trim computers that worry over such nonsense...
Snif Snif...I think maybe you've eaten too much broccoli today...

Really though, I think the front sensor is responsible for tuning information, the rear one is solely to inform of catalytic convertor health...I.E. if the efficiency of the converter falls to below 95% efficiency, you get a code... simply letting you know your dryer sheet is losing it's ability to freshen up your flatus...so, the rear sensor mod= NO harm NO foul in my book.

I'm trying to do this as I just had a P0420 pop up and I can't seem to find the rear O2 sensor. How far back is it located? I'm not sure if it differs year to year but I'm in an 01.

This is what I'm guessing to be the front..


And the rear?

@MattJakobs :

It's right over the crossmember in the picture buddy......


You also have a Cali Emissions 4Runner, just FYI.

URD in general is a huge crock of shit.... Their simulator is exactly the same as this, but is just $98 more. I am so tired of people praising their crap like it is the last word in tuning. Their statement doesn't even apply to your model year anyway (may want to check, could be wrong 99' was a funky year) since it has narrow band front and rear!

Edit: Just noticed this was a bumped thread, but my statement stands...

My '99 is Cali emissions and has a wideband front sensor.

There seems to be some confusion here. Only the front o2 sensor is used to adjust the fuel mixture. The rear o2 sensor is only used to monitor the performance of the cats. Removing or simulating the rear o2 sensor will not effect performance or emissions at all. Simulating the rear o2 sensor will let you remove the cats and not get a CEL. Only the California emissions 4Runners have two cats. Removing the cats on a stock engine will not gain much if any performance, but it's a way to save a bit of money if replacing the exhaust system, especially for those who live in states with no annual vehicle inspections. Outside of certain cities, I would have zero concern about any law enforcement even knowing what to look for as far as a catless vehicle is concerned.

Just curious why you would use a 250V capacitor on a 5V circuit? Was that just the only one they happen to have in stock at radio shack? A 5V or even 10V capacitor would be plenty for this application.

Also just FIY for anyone interested in doing this to your O2 sensor: the specific rating of the capacitor/resistor isn't what allows this circuit to work; the key is to match the capacitor/resistor to each other so the transient response of the RC circuit has the appropriate time constant. In this case a time constant of 1 second appears to do the trick. So any RC pair with a 1 second time constant will function the same. (i.e. 1µF/1MΩ, or 1mF/1kΩ, etc)

Sounds like you know more about the "how it works" than I do. I didn't come up with this mod myself, I've just seen write-ups for it on different sites. Since many of us suffer from the dreaded P0420 code on our 4Runners, I figured I'd try it here, make a post about how I did it, and share my results.

Darn EE's and their RC circiut and time constant witchcraft!

Best bet if you don't live in Cali, stay clear of the vehicles. We tell our buyers not to buy them. They rarely accept aftermarket cats and when they do your on barrowed time. You can try what we call an 02 bung or spacer, it doesn't delete it but moves it out of the stream enough to clear code in most cases.

In Tx having a missing cat can lead to the state not letting you register your vehicle and fining you. An inspector letting it pass safety can get a fine and suspension of license's driving and inspector. Even diesels are stamping codes on the valve covers as to what cats are factory equipped.

But as we all know it easy to get an inspectors license with little to no knowledge of vehicle mechanics. Thus resulting in high turn over of inspectors due to officers randomly running sting ops for lax inspectors.

I hope this make it mo better for grammar and English lit folks, I was only able to attain electronics and mechanical eng degree's. I hated English !!!!

might want to reread these last two paragraphs for grammar mistakes. They got kind of hard to follow.

Might want to re-read all this for mistakes.



There are plenty of people here that have used an aftermarket cat, have passed inspection, and have been fine for years, if this was the route they chose to go.

If you live in Cali, I would say to stary clear, simply because Cali does the Sniffer test, and while the light might not be on, the exhaust can still cause it to fail, from my understanding (I don't live in Cali, nor whould I ever want to).

A O2 "spacer" is going to move it out of the gasses? How? Hose gasses are still going to be surrounding the O2 sensor. those gasses are going to fill every inch of that tail pipe, so by moving the sensor "out of the stream", you're really just moving the sensor up. Not doing anything at all really.....