Both are correct, It's illegal to remove or replace a fully functioning cat UNLESS vehicle is over 50k miles and there is a documented reason for replacement (those in the rust belt are in luck, just make sure the right parts break "due to rust" when you replace your exhaust...) under 50k miles and it can only be replaced if damaged.
On the down side, as far as Blazers go it is totally illegal to switch over to a true dual exhaust even if you install dual converters, has to stay single cat. Your only allowed to swap to duals if the vehicle model for that year or later years was offered with a dual exhaust with dual cats which as far as I know the K5 never was. Not sure about the newer Yukons etc but those wouldn't count anyways..... It's also illegal to remove the cat from any vehicle even if the vehicle is never to be used on-road again (ie: making a full-offroad non-street legal vehicle, you would still have to keep the cat on it or your breaking the law)
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BS.
If the vehicle in question has more than 50K miles, it is perfectly legal to replace the cat.
You can also legally replace it, if it is damaged /forums/images/graemlins/whistling.gif
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That is not true. It is illegal to remove a functional catalytic converter.
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