my buddy has been tryin to pass smog. he bought the cat and both o2 sensors. and now the only thing he wont pass is nox? what is causing him not to pass? the EGR valve? also does anyone have any tips for passing smog?

 

Its pronounced NOS

What kind of car/truck does your buddy have? what mods are done to it?

 

I didn't even think NOx was a part of IM240, noticing you are from california of course you are a year or two ahead of the rest of the country. What NOx is a chemical formula that is created when cylinder temps reach above 2500* I belive its a nitrogen molecule combines with X amount of oxygen molecules above 2500*. Your gonna have to let us in on some information about the truck.

 

95 altima
all stock

 

Oxides of nitrogen (NOx) is what causes the visible brown smog. N2 and O2 exist in the air as inert gasses and don't react by themselves, they require heat energy. So, your buddies car is generating too much heat. Is it running lean? Look for causes of the heat.

My understanding was that CA was not going to fail cars for NOx until 2004 because they were collecting data to set their threasholds for passing tests. Maybe SoCal is different because you were using the dyno before we were.

 

Texas will allready fail you on NOx.
It very well maybe to lean, once that is taken care of you should change the oil and filter and drive it on the freeway at 60 mph or so for 10 miles.
If you warm up the exhuast/engine, it has a better chance passing then if you just start it up and take it to the testing station.