Ok, here's the scenario: 95YJ 2.5L (with 4.0 throttle body)

NOxlevels were over 1500ppm on the first test. Since I've seen this before, we changed the catalytic converter. Sure enough, you could see daylight through the old one. Drove about 3 miles and retested. It passed the low speed test, but failed the high at just under 1300ppm on NOx. So they said drive it about 10 miles. So I did. The third test, levels jumped back up to almost 1500ppm again. Put some Lucas injector clearner in it, and drove about 100 miles over the weekend. Had a fourth test done today. NOx is now at nearly 1900ppm!!! So I took it to another station. Their machine shwoed about 1300ppm on the low speed, and 1650ppm on the high speed. Still failing emissions. The other two tests they run show minescule amounts compared to the norms at both test stations. So tonight I replaced the O2 sensor, and will have it checked again tomorrow.
I'm pretty much stumped, and so is everyone I talk to. Only idea I have is they sold me a bad catalytic converter. ( It was new, and I watched them install it.) Has anybody had similar problems and know what the heck is going on? Or am I overlooking something simple here?
Thanks gang!

Is it showing any codes?

DOH!!! Hadn't thought to check that. Gimme a few minutes to run out front and see.

Got a clean bill of health. It flashed 55-"End of display sequence".

then Shot it and build the XJ.

Thought had crossed my mind Mark! But....I finally talked to a mechanic that knew what was going on. The fiirst question was "Do you have a leaking exhaust or intake manifold?" Well, yeah, I just happen to have a cracked out exhaust manifold! So he recommended replacing it and changing the O2 sensor and spark plugs while I'm at it. Cracked manifold equals extra air in exhaust system, telling the O2 sensor the engine is running lean. So the computer cranks the mixture up, and running too rich cooks the catalytlic converter, and also causes the high NOX levels.

So.....the header I got from 4x4Rockshop around 1 year ago is finally getting installed. Hopefully this should put an end to burned out cat's and failing emissions.

It's a miracle!

AFter installing the header, replacing some worn through vacuum lines, a new O2 sensor, and new spark plugs, the NOX levels are now down to (drum roll please!)


60 on the low speed check, and 86 on the high speed check. So the YJ is all street legal once again. I get a feling my gas mileage just might go up a bit too.

Congrats Dennis, sometimes you have to throw the kitchen sink at these things to get them to pass emmisions, or to fix any problem for that matter. Atleast your gas milege is up.

I knew *something* was odd with it, just couldn't put my finger on it. We park the wife's XJ this week to start fixing a few odd and end things on it. And I'll be using my YJ for my daily driver for a bit. So I'll get a chance to see just how much my mileage comes up now that the engine is getting the right mixture thrown at it.

its not a miracle, you could of had a worn, cracked manifold, bad o2 sensors, bad plugs, or your vacuum lines were giving the computer a hard time.. its called maintence.