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The case relearn is a crankshaft position variation relearn. If you don't do it, you may get a check engine light because the pcm is reading false misfires because it hasn't been learned to the crankshaft position variation. Any time you mess with the pcm, crank position sensor, crank, this should be done. Sometimes you can get lucky and it won't set a check engine light, but most times you have to do the relearn. If there is any GM dealers there with a tech2 they can do it for you.