Apple describes what users may encounter:
After you try to update or restore your iOS device in iTunes on your Mac or PC, you might see error 53 in iTunes and "Connect to iTunes" on your device. Error 53 appears when a device fails a security test. This test was designed to check whether Touch ID works properly before the device leaves the factory, and wasn’t intended to affect customers.
For anyone who experienced error 53, Apple has released an update to iOS 9.2.1 to allow you to successfully restore your device using iTunes on your Mac or PC. Use the steps in this article to restore and recover your device. If you believe that you paid for an out-of-warranty device replacement based on an error 53 issue, you can contact Apple Support to ask about reimbursement.
What you should do:
1. If iTunes is open on your Mac or PC, quit iTunes.
2. If your iOS device is plugged into your computer, unplug it.
3. Make sure that you have the latest version of iTunes.
4. Connect your iOS device to your computer with a USB cable.
5. Open iTunes and select your device.
6. When you see the option in iTunes to Restore or Update, click Restore.*
7. When you see your iOS device's Hello screen, follow the onscreen steps to set up your device.
● If you previously backed up your device, you can set up from your backup.
● When you see the screen for Touch ID, tap Set up Touch ID later.
8. If Touch ID on your device didn't work before you saw error 53, the feature still won't work after you restore your device. Contact Apple Support to ask about service options for Touch ID.
You can download the new version of iOS 9.2.1 from here:
● iPhone Firmware Download Links
● iPad Firmware Download Links
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