is Black magic. I knew I might have had a bad O2 sensor so I looked at it and it was Black with soot. So I switched it with new one, and I still failed emssions.
I tried to be analytical about it. I read all over the Internet about it. High NOx is most often a result of a bad AMASS, and I even put an oscilloscope on mine and I thought, "I found it!"
The AMASS was reading in the 2.7 Volts range at idle. I went and got a spare AMASS from a yard and switched it. It read correctly at 2.5 Volts. I drove the car and checked the O2 sensor. The O2 sensor was clean as a whistle! i was thrilled.
I went in and Failed again!
So now I have to switch my catalytic. Luckily i have a warranty on mine. Nothing makes sense. The car is pumping out 2x as much NOx as it should be and my gut feeling is a CAT wont clean it up that much.
Also even if my fuel pressure was low due to a worn fuel pump, I dont think it would be low enuff to raise the NOx that much! To do that you probably would need 25 psi agianst 40 psi. But the car runs beautifully.

Sometimes you get into Paralysis by Analysis. Dont make it too complicated or overanalyze.
Simplify.
My gut feeling is the best way to solve it is to take it for a 1000 mile drive and wait for something to fall off. Whatever it is, is whats causing the problem....

To DKVEURO:: Being a good parent is even more BLACK MAGIC and harder than fixing the cars...again you have to go with the Gut Feeling and not paralysis by analysis---no one will take care of your kids in this world except themselves and your self, so you have to go mid range, don't give them to much now, but save for them later....