This could go in 900 as the car is a 93 940T, but it's the same as my 740T and the issues cross over.
I got the car 2 years ago and it just passed for NOx. I thought I had the results from 2 years ago but haven't found them yet. I want to see if the limits have changed, and how my numbers compare. Other numbers were in the middle of allowable.
Various things happened, changed a lot of ignition parts, fuel parts, and I was concerned about passing when required which was this month.
I got a pretest which showed around 825 NOx at the 15 and 25 speeds. Limit is now 549. I had the exhaust checked, no leaks, new 02 sensor a few months ago. Then the car died and it was thought to be an old fuel pump and an original filter. As I have written about, the car ran quite well after, smoother and more powerful, and seeming to get better mileage.
I was going to try some other things but it ran so well I thought I'd pretest again; this time high 400's range.
So today I went for the actual test, and passed.
-------------HC-------------CO-----------------NOx

15mph,___max59 meas26___max.50 meas.07___max 549 meas537
25mph,___max35 meas15___max.47 meas.13___max 502 meas476

Conclusion; that the old pump and Very old filter may have leaned out the mixture just enough to make the NOx go up.