How do I fix my iPhone Face ID

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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 the only option that you have is to take the device to...

Take your device to an Apple Retail Store or Apple Authorized Service Provider, or contact Apple Support:

Find Locations - Location Selector - Apple

Contact Apple for support and service - Apple Support 

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.

Do you have a disabled Face ID alert on your device?

Please read the "What to do Next" section in this link, if nothing works as per the instruction given in this link --> If an alert says Face ID has been disabled on your iPhone or iPad Pro