Although Argentina and other South American countries will not care about the lack of a registration STICKER, but if the registration DOCUMENT you are carrying has an expiry date on it which is past, you will have difficulties at borders. And if you have a carnet, your vehicle registration needs to be valid for the period of the carnet.
A carnet is not absolutely necessary for South America, but will make the process much smoother at borders, and may save some customs fees as well. If your trip involves other countries (Asia, Africa), which require a carnet, you should have it validated for South America as well. If you're only doing South and North America, probably not worth it since Central and North America don't require it. Worth checking at your local auto association as to costs.
Good luck and safe travels!
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