Galaxy S4 Sim ejected problem

I've got several S4 boards here with the intermittent "sim ejected" problem, and one that just plain doesn't recognize the sim card at all. I know that it's not the reader or the sim card, and seems to be a common problem with these phones. I've also seen several comments in threads on this suggesting that it's a bad IC, but no one says which IC! I'm good at soldering, and can probably replace it if I just know which one to replace. If anyone knows I'd be forever grateful!