This is very interesting. I can not pass NOX in So Cal. Here is some info on my 1989 YJ

. 4.2L engine rebuilt Aug 2008 with 4.0L head. I am using Champion RC9YC @ .040 gap. After the rebuilt I used Autolite 3923 but changed them after talking with Hesco. Hesco recommends RC9YC for the 4.0L head because the 3923 plugs run hot.
. Installed the Mopar MFI conversion kit from Hesco. Very sweet system !
. 4.0L header from Banks. Excellent !
. New radiator in 2007
. New universal CAT install in 2007 just before smog test. And yes my NOX reading were higher than average in 2007 (503ppm (1020max) @ 15mph and 596ppm (880max) @ 25mph). In 2007 my YJ had the OEM BBD carb and new EGR.

Now it is 2 years later and I need to smog test again. No problem I thoughtà.rebuilt engine; 4.0L head; Mopar MFIàwhat could go wrong ?

Last week NOX test resultsà.
2039ppm (1020max) @ 15mph
1686ppm (880max) @ 25mph

Today NOX test resultsà.
1330ppm (1020max) @ 15mph
1352ppm (880max) @ 25mph

Better, but still FAILED. Now you askà.what is different from last weeks testà.
. Orange Engine told me to use Chevron Delo 15-40W oil for break-in period. I used Delo for break-in and second oil change. Delo is diesel oil and should not be used in gas engines. Yesterday I changed to Royal Purple Synthetic 10-40W. Expensive ($8.99/quart)
. I changed to Champion RC9YC (recommended by Hesco). I had Autolite 3923 plugs.
. Hesco told me to check to see if oil blow-by thru the CCV system into the intake. I checked and noticed a little oil residue below the throttle body. I even checked for oil on spark plug in cylinder 5 and 6. These plugs were very dry. I figure I could rule out oil blow-by. Oil in the intake would create a lean mixture and high combustion chamber temps. I put a small clear fuel filter to catch any oil blow by, if any.
. Fuel pressure is OKà.48-51psi (per Hesco). Taped gage to windshield and ran it uphill under loadàno changeà.around town vs. under loadà

After all thisàI will look into getting a new CAT as described on June 6th thread. Any comments would be useful.

Replace the CAT.

Had an 89 4.0l Cherokee fail due to high NOX - replaced CAT; passed
Had a 94 Wrangler 4.0l fail due to high NOX - replaced CAT; passed

BTW, also replaced O2 sensor...

Thanks. The O2 sensor is new. It came with the Hesco / Mopar MFI kit I installed Aug 08. Only about 1,000 miles since then.