As far as I can remember Carson City does not require emissions testing at all. Only residents of Washoe and Clark counties are required to Smog test.
And btw, CC is not a part of Douglas County. It is in fact a city without a county, also known as an independent city. It was at one time encompassed by a county named Ormsby County, but they did away with the county government there and it is simply a city without a county. I have gotten into many an argument with Renonites over this as they will typically tell you that it is Carson City County but this is simply not true.
Here is a link if you don't believe me:
Carson City, Nevada - Wikipedia, the free encyclopedia
And this isn't unique to Nevada. St. Louis Missouri is another example of a city without a county. Hope this info helps.
PS: No county gov't means no county taxes!