The Nissan Path

Hello,
I have a '98 pathfinder with 180000 miles. The state of Colorado requieres 2.5 or less. The truck did not pass with a 3.56. I dont want to waste money on changing the catalytic converter and then have it not pass. The only code that I can read is the knock sensor failure, otherwise it runs great and the check engine light is not on.
Any ideas?
Thanks,
Richard

Check your EGR system. If that's working properly, I would recommend using a top engine cleaning system, such as Motorvac.

Thanks for the answer. I checked the EGR valve with the mitvac pump and it is functioning correctly. I dont know what the official procedure is to check the EGR system. Maybe someone can help me with that. Do you have any idea at what mileage one is supposed to change the catalytic converters?

There is no mileage interval to change the catalytic converters. You replace them when them become inefficient (in which it will throw a three way catalyst trouble code if it's an upstream converter) or when they become restrictive, which will usually result in poor acceleration, potential setting of miscellaneous trouble codes and can be checked with an exhaust backpressure tester. High NOx emissions are caused by excessive heat or excessive pressure in the combustion chamber. I remember taking a training class when I was with Nissan and the trainer telling us something like 90% of emission failures are due to high NOx emissions and 90% of those vehicles can be fixed by a top-end engine cleaner.