so ive searched... and searched.. for answers to this question!

heres the low down, i purchased an 01' xj from a guy that lived in an area that emissions was not required (did not know this at the time) and i tried to pass emissions and failed. the check engine light doesn't work at all, and is commanded on. its throwing codes po300, po161, and po158. the later two are for the rear o2 sensor. i went to napa and asked for a rear o2 sensor and got one. got home and went down and looked, found an empty plug hanging over the transmission.. is this the right location? there is no spot in my exhaust to install it so i'll have to fab one. Also, while underneath i noticed a spot just below where the y pipe comes together that looks like a sensor should be but isnt and it is plugged..

need to know how many o2's i should have, and where the heck do they go?

 

You should have 2 o2s... One in the downpipe (this is the pipe after the exhaust manifold) and one right after the catalytic converter. There should be bungs in those locations. But since there isn't, I believe there is a way to weld in bungs. If you don't know how to do that, any exhaust shop will be able to, for fairly cheap too.

Take a picture of the plug hanging over the tranny

 

X2^ I saw mentioned in another thread the idea of using one of those spark-plug anti foulers. The sensor I have sitting right here has larger threads than a normal plug though, more like the ones on some older Ford plugs I've seen. I guess they cut it in 1/2 and welded the female threads there after the cat. I believe the plug has three wires.< Correction. Not sure what you have. Some 90's Modl's had 4, ground, heater hot, sensor feed & sensor return....something like that!

 

I will post pictures later tonight, but I looked up underneath again and it looks as if I have three sensors all on the manifold.. nothing at all on the down pipe except the blocked bung.

 

The first "upstream" 02 sensor goes right there close..mine is in the exhaust manifold. It's the one the engine uses to "trim" ? the fuel mix. Important for mileage and just runnen right. The second "downstream" sensor is after the catalytic converter. It just monitors the cat and "sets a code", turns on the light, when it or the cat isn't working right.

 

so here we have the pictures, first of the plug for my rear o2, second, the sealed bung, and third of my exhaust manifold.

 

and i understand the basics as to how it all works, first one trims the mix rear is supposed to tell how the cats doing, but i also keep hearing that the lack of a rear o2 could also be causing the po300 code and the rough idle caused by the miss'. the plug for the rear o2 of course is hanging near my front drive shaft and the cat is another couple FEET away from it. the wiring is no where near long enough. is the cat in the wrong spot or should i just splice in some longer wiring?

 

A half hrs worth of text just vanished. That's a goofy setup in your pic. check post #2 here . gotta go cool off. http://www.naxja.org/forum/showthrea...ht=sensor+test

 

The rear O2 sensor has some influence over the mixture, but not near as much as the front one.

It's not going to give it a rough idle, but it may hurt mileage a little.

If someone could put up some clear pictures i would be helpful.

If there's no ready place to put them, a muffler shop can install the bungs needed for cheap. Unless you go to a chain store like Midas etc, they'll rape you.
Find a small independent one.

 

The plug hanging there is for the downstream o2, a bung can be welded into your pipe where your cat was. The bung in the downpipe is for a sensor also. What has me confused is the 3 sensors in the last pic, they look like o2 sensors in your manifold??? I am not familiar with the 01' but I can't make sense of that.

 

Not positive, but I think sometime in 2001 they switched from 2 to 4 sensors... Here's a picture I found

 

I think you are probably right. My '02 4.0 TJ has 4 O2 sensors in a configuration much like you've shown in your pic.

 

2000 model year actually. Though TBH I thought there were three pre-cat O2 sensors, and a final one before the muffler.

 

Mr Towers, my cat is right beside my transfer case. I don't see a problem with extending the wires, unless you want to invest in redoing your exhaust. They like it close to the engine to help it burn hot. I install the shrink tube and slide it aside, so after the splice is soldered you can then slide it over and shrink it. Then zip ties. You shouldn't put one of those "dummy" devices on that just to pass smog, that's illegal.

What salad is saying gives hope that you might have something there that will do. I might fix that bulb, get the light working (get that post cat wiring hooked up), then deal with the codes.

Then, the "rough idle caused by the miss", would be front and center. Plugs, wires, cap, rotor, vac leaks, Clean IAC & TB.

 

Do you have the 2 pre-cats like in the picture?
Then they went to dual pre-cats they also went with 4 o2 sensors.
California got the first ones, then other states followed the next year.
Behind the 2 pre-cats there's another bigger cat - looks like a muffler - no sensor after that. After that big cat, it goes to the muffler.

Previous to dual pre-cats there was only one bigger cat, 2 sensors - one before and one after.

The sensors after the cats do have a small influence over mixture - but over a longer period of time vs. the close ones have an almost immediate reaction.

The pre-cats - like in the picture - are only a platinum screen that glows red hot and further burns whatever didn't burn in the cylinders.

They seldom go bad, but often get replaced due to stupidity - they are very expensive!
When you take them off you can see right through them. Either the screen gets a hole in it (replace,) or it gets plugged with carbon (soak in lacquer thinner.).

As far as I know, the 2 pre-cat/4 sensor versions all had the coil rail - DIS ignition, - no distributor.