ios 11.2.1 touch id iphone 6s plus not working

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Update please to 11.3. But there wasn't no bug in 11.2.1

Did you have an non original part (screen and/or touch id) replace in another service (not Apple Service)?

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I am throwing the BS flag on that play. Yes, there IS a bug, several hundred people have said so. iPhone worked before the update. Does not work after the update. I would say there is a very high possibility of causal coincidence here.

It can't hurt to update, hopefully. Since my phone is now crippled anyway. And yeah, my iPhone 6s was paid for in cash, unlocked and on AT&T.

DLC

You are of course entitled to an opinion, as all of us :-) But "hundreds" of people as compared to roughly 500 million iOS users are not that big of a number. In a past case where Apple did have a bug you've seen millions of comments in hours/days from that event.

Thus being said you didn't respond if the iPhone had some hardware replacement in a non official service?

Also did you try hard resetting your phone (backing it up before), set it up as a new one (not restoring from backup) and check a couple of days to see if the problem still happens? This might help you see if there is a problem within the backup (as some processes can get corrupted during backup - see also this topic on battery drain after iOS update Re: IOS 11.2.5 battery drain ) of it is a hardware issue.