http://www.hptuners.com/order.php
You can do more than clear or check codes.
This is the same as the shops use to do your car, they buy multiple credits.
I am not sure of the exact numbers but different spec LS engines (and auto) use different quantities of the credits.
You will also need a laptop, as I have said I do not have one yet but my understanding is that they are pretty easy to use and they walk you through all changes you make.
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Cruising in 5th
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Never be afraid to do something new, Remember, Amateurs built the Ark: Professionals built the Titanic.