Failed smog test for high nox

GregBelleville

TJ Enthusiast
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Sep 19, 2022
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Southern California
Hi all, I live in Socal and am due for my first smog test since owning my TJ, which is a 99, 4.0. it's got 192k miles on it and no CEL codes.

I took it in this morning and it failed due to NOX being too high. The tech recommended looking at the O2 sensor as a possible cause, and I know the ntk/ngk o2s are preferred but hard to find anymore. Does anyone have suggestions on where to find one of these or a suitable alternative they've tried that worked well?

Anyway, I'm also looking for advice on what other possible things I should look at and test for that could cause my NOX reading to be high?
 
A bad O2 sensor normally causes it to run rich and a high NOX often indicates the mixture is too lean. I looked around and confirmed a defective catalytic converter seems to be most likely where a TJ is concerned.
 
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A bad O2 sensor normally causes it to run rich and a high NOX often indicates the mixture is too lean. I looked around and confirmed a defective catalytic converter seems to be most likely where a TJ is concerned.

Thanks for the reply Jerry. So you think the cat is the most likely culprit? Is there a way to test a catalytic converter to confirm before I replace it since they're so expensive?
 
Thanks for the reply Jerry. So you think the cat is the most likely culprit? Is there a way to test a catalytic converter to confirm before I replace it since they're so expensive?
I'm not sure if that is something that can be tested for Greg. You might try Googling high NOX to get more information, just ignore all the suggestions for a bad EGR valve which the TJ doesn't have. Other causes can be bad engine timing (hard for an unmolested TJ to have) or a carbon buildup inside the combustion chambers which usually only happens when the engine is driven too conservatively for too long.
 
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https://www.wranglerforum.com/threads/smog-help-hi-nox-this-was-the-fix-3-jeeps.1569817/

I found this post on the other forum from an actual smog tech here in Socal giving several causes for high NOX in jeeps. He gives several helpful tips on possible culprits for high NOX which I'm going to look more into:

First example he provides, he got the Jeep, a YJ to pass by replacing the thermostat:

1st yj 1994,4.0, 5spd fails for high nox.. 2,000 and HIGHER 15mph, 25mph. HC, CO all low well within limits. vehicle has 187,000 miles on it. perform a chrysler top end cleaning (better version of sea foam) nox drops to 1,000 NOX at 15, 880 at 25. Notice the temp gauge is running 210 with long swings to 150 and back. Client says they have never replaced thermostat. Replace thermostat, engine steady at 185-190. passes test 15mph 450 NOX, 13 HC, .07 CO, 25mph 550 NOX, 16 HC, z.02 CO

Second example, exhaust leaks prior to the catalytic converter. New exhaust system just installed but failed and he found that until the exhaust was welded the clamps were leaking. I need to check for possible exhaust leaks ahead of my cat.

LETS TALK EXHAUST LEAKS.... if you have 3% oxygen entering the exhaust system ahead of the cat in ANYWAY, cracked header, burned exhaust gasket, ETC. the oxygen renders the cat useless for clean up NOX. Remember the new exhaust? It was a really nice kit supplied with exhaust clamps.... Weld entire exhaust into place, after running multiple tests we found a HIGH amount of oxygen in the system, putting our hands up to the exhaust you could feel an exhaust pulse no matter how tight the clamps got... Once welded. NOX was 100 and zero (15,25)

Third example, non ngk o2 sensor.

3rd Jeep. 4.0, 5spd, customer brings jeep in, jeep fails NOX 1100 at 15mph, 1970 25mph, HC and CO all relatively low HC 30, CO .07. Customer says he put an 02 in 4 months ago to get better gas mileage, has since gotten WORSE gas mileage and has to run 91 to stop pinging. Customer Sea Foamed 20,000 miles ago. last 2 smogs vehicle has always passed no problem. Customer replaced 02 with factory chrysler unit. There is something to be said cause the factory sensors are no longer working properly for us, they are all lean biased in the last 4 jeeps I have had to put 02's in. I found the original supplier use to be NGK, but sometime in the last 2 years the 02 sensor itself changed in appearance and ever since then I have had 02 problems with new out of the box units. All I did was through in the NGK 23023. HC 13 CO .02 NOX 650(15) 400 (25).
 
I found all four o2 sensors on RockAuto about two weeks ago, I snagged them up. They may still have some available. Just checked still there in case you want a set for the shelf.

IMG_1109.jpeg
 
I'm curious though if your Jeep was purchased new in CA and if so is your equipment different than the 49 state models?
 
LOL, yeah I'm going to try and figure out if I can fix the Jeep and pass smog, but if I can't, can my family and I crash on your couch?

The thing I like most about CA. is living next door so I can visit whenever I want. Love the San Diego area and up the coast North of LA. Next trip the plan is to spend time along the coast between Solana Beach and Oceanside in a nice RV park on the beach and take day trips in the TJ.
 
The thing I like most about CA. is living next door so I can visit whenever I want. Love the San Diego area and up the coast North of LA. Next trip the plan is to spend time along the coast between Solana Beach and Oceanside in a nice RV park on the beach and take day trips in the TJ.

I do Glamis every February, come on out!
 
Yeah 192 is low. Is it possible a lower temperature thermostat like a 165 or 180 degree was installed instead of the correct 195 degree model?

It's possible, I've owned this Jeep for about 2 years, and haven't replaced the thermostat in that time. Could the high NOX and the low engine temp be possibly related? From what I've read, high nox is caused by high combustion chamber temps, so I would think the engine running cooler would be better for reducing NOX?