Other possibilities for High NOx:

Malfuctioning/Inoperative EGR valve, Out-of-calibration MAP sensor, plugged injector, low fuel pressure, vacuum leak, advanced ignition timing.

Any of those malfunctions will cause higher internal temperatures, hence increasing NOx.

__________________
1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car
609 Certified