If it is the O2 sensor, it should store a fault code. That car has 2 O2 sensors. What did you do to reduce the NOx, replace the cat?

__________________
Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record