Rob, Thats the thing, I can check 02 sensor with a Computer, And The cat with the Actual Emission machine, For me it is the same thing, I was Replying to Brandons Statement when he says, A cat doesnt affect The limits like that which is for sure not true, And A EGR is the main cause of a High Nox Which I am saying is not true for every car, and Yes EGR should be checked first because it is the easiest