Alright well i was gone for 4 months attending Navy basic and MA "A" school. While i was gone the car sat. When i got back the registration needed to be renewed and it needed a smog. The results came back with really high nox (3 times legal limit.) I replaced the cat 2 years ago when i last got it smogged. The car runs good and doesnt through any codes. Its a 90 civic with a sohc zc, header, and intake. Other than that it is pretty stock. What could cause this issue? I appreciate the help

 

your cat is bad...i had a cat go bad on me after 1 year since it was el cheapo...also check to see if your o2 sensor is still good...!!!