Use a Vacuum guage to check the the Vac at idle and buy the kit with the vac pump so you can check the EGR valve function per the FSM. Esure the EGR ports in the intake are clear of carbon deposits.

If thats all good to go, You might try running a "Sea Foam" treatment to clean carbon deposits from the combustion chambers. Deposits can raise temperatures in the CC causing high NOX levels.

Changing the O2 sensor is a good idea as well especially if you dont know when it was last replaced.

I went through this with my 89 when the NOX was off the charts and was flagged as a gross polluter. I changed the 02 sensor and retested and cut the numbers in half but still failed. Once I got the EGR system squared away and ran a sea foam treatment it passed easily. HTH

99 Gen 2.5, fixing blown head gasket
89 SWB- 33's, ARB Front locker, SR rear locker/axle, SR F brakes, winch, WST Offroad Armor all Around, 2.85 Aussie T-case Gears (SOLD)
Sold: (2) 95 SR's, 86 SWB, 90LWB, 91 LWB
-Can Change a timing belt in my sleep..