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If you still need help, reset Face ID, then set it up again. Go to Settings, tap Face ID & Passcode, then tap Reset Face ID. Next tap Set up Face ID to set it up again.
If Face ID isn't working on your iPhone or iPad Pro
If your iPhone has a history of Camera replacement / Repair / Service from an un-authorised service provider then it is possible that Apple may refuse to service your iPhone if it has been previously serviced by an unauthorized service center.
Get a Genius Bar Reservation and Apple Support Options - Apple
Contact Genius Bar.
Manage Reservations - My Reservations - Apple
Parts and service history
With iOS 15.2 and later and iPhone XR, iPhone XS models, and later—including iPhone SE (2nd and 3rd generation)—you can go to Settings > General > About for the parts and service history of your iPhone. You can also find out if the battery has been replaced. If the replacement was done using genuine Apple parts and processes, "Genuine Apple Part" will appear next to Battery.