I failed the NOX part of the Calif. emissions test. NOX at 15mph was 573 and at 25mph was 395.

Does anyone know why this would be and what causes it? It seems weird that it would pass at 25 but fail at 15.

 

If I think I knew what NOX is I would say the your air/fuel mixture is screwing up. Either too lean or too rich on the fuel. What mods are on your car.

 

are you gettin a CEL? i passed my smog with stock intake, warpspeed y-pipe and magnaflow muffler last year with ghost 0304 (knock sensor) CEL ran lean all the way.

i say manually pull the CEL, and make sure all sensors are working. and most importantly, make sure your stock cat is working properly.

 

I have an intake and y-pipe. My CEL is off but I haven't tried to pull codes yet. NOX is some type of Nitrogen Oxygen gas. Something like NO3... Anyways, I found out that the sole job of the EGR is to lower the NOX and keep the engine running cool. I checked the EGR valves, solenoids, and made sure there was no vacuum leak. Everything seems fine. I think my Cat has gone bad.

How long does a stock Cat usually last with mods?
What's a good recommendation for a Cat?

 

Could be the cat.

Go OEM when replacing cats. Aftermarket (Part store cats) are junk and aren't as "clean".

 

How far away is the testing facility? I mean, did the car have adequate time to really get up to operating temp or was it just a 5 minute drive?