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).
