Generally if one of the components of the Face ID system has failed, only Apple can replace that part, as they are specifically paired to that logic board and Apple is the only one who can do that pairing. Rather than repeat my whole post on the subject, here's a link to it that explains the whole situation.
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[post|733853|Face ID not working - iPhone XS Max - iFixit|new_window=true]
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[link|https://www.ifixit.com/Answers/View/733853/Face+ID+not+working#answer733860|Face ID not working - iPhone XS Max - iFixit|new_window=true]
The Face ID issue and the replacement screen issue really are two separate problems; as @imicrosoldering has pointed out, new screens won't affect the Face ID function as long as you use the original earpiece/illuminator assembly from the old screen.
Hi Jenni,
Generally if one of the components of the Face ID system has failed, only Apple can replace that part, as they are specifically paired to that logic board and Apple is the only one who can do that pairing. Rather than repeat my whole post on the subject, here's a link to it that explains the whole situation.
[post|733853|Face ID not working - iPhone XS Max - iFixit|new_window=true]
The Face ID issue and the replacement screen issue really are two separate problems; as @imicrosoldering has pointed out, new screens won't affect the Face ID function as long as you use the original earpiece/illuminator assembly from the old screen.