Look for a vacuum leak. Im in that same predicament in my new car. It sucks. My nox is up in the 2700 but I ran it once while the motor was almost cold and it only hit 950 (still failed). A mechanic told me it was a vacuum leak, and they expand when the motor is hot thats why the level is higher on a hotter motor. I already changed 02 sensor, and I don't have EGR either