Its a 95 integra with 190k miles. I had it smogged 2 times last month since i got the car 2 years ago. The car is bone stock except for the cheap *** cold air intake that came with the car when i got it. Since owning the car i have replace the wire, spark plug and thermostat. First test i did nothing to the car except for oil change. Everything pass except for the nox. Nos failed alot on 15 mph only.
The second time i had tested i did the timing and brought it down to 15 instead of 17. But still failed the Nox, i think is the same amount. What do you guys make up on this. I really dont want to change the cat and i dont think the cat is the problem.

On my research, Nox is due to high temp on the combustion chamber. Im running lean on 15 mph? oxygen sensor?
15 and 25 mph don't have that much difference on the RPM (15 is @1800 and 25 @ 2000rpm). I guess only difference is the throttle opening between 15 and 25. Maybe 25mph the throttle open abit more than 15mph and giving the engine more fuel but i dont know. I will definitely clean and check my injections and manifold on my dayoff.. thanks...