The Fraser Institute, a Vancouver research group, has done a study that observes that with recent cars and their longevity of tune remain as about as good as it gets. Older cars eventually die and no longer pollute.
The conclusion is that on such testing the cost is greater than the benefits.
For this, as well as a certain mission of civil disobedience--I don't run cats.
In order to get passed, the shop put two NOX cats in series as one pipe, and when it is time the two resonators are removed, the cats put in and the computer is re-tuned.
Then after getting passed all is re- set for performance.

Absolutely useless exercize.
