On the covered Apple iPhone or iPad, 53 means Apple's article lists 53 with the errors that might mean a hardware issue, and links a separate Apple article for what to do about error 53. First, follow the separate Apple error 53 article that Apple's page links.
Try in this order
0 of 8 completedWhat to do first
ReversibleFollow the separate Apple error 53 article that Apple's page links.
Correct only the documented fault domain
ReversibleOf the hardware-group errors, 53 is the one Apple gives its own article, because the cause is specific: the Touch ID sensor no longer matches the device. Reopening iTunes or swapping cables does not change that.
Understand what caused it
ReversibleApple's page says iOS checks during an update or restore that the Touch ID sensor matches the device. The error was mostly seen after an unofficial repair replaced the home button or its cable.
Get the sensor serviced
ReversibleIf Touch ID did not work before you saw Error 53, it will not work after a restore either, and the phone needs service.
Take it to Apple or an authorized repairer
ReversibleBecause the error is about the Touch ID sensor not matching the phone, restoring the software will not solve it. A repair that replaces the sensor with a matching one, done by Apple, does.
Check whether Touch ID worked beforehand
ReversibleIf the fingerprint sensor was already failing before the update, restoring will not fix it; the sensor or its cable needs service.
Update the computer's software
ReversibleApple's page notes that error 53 can show on a Mac in Finder or on a PC with iTunes, so make sure the computer itself is current before trying again.
Try another computer if it cannot reach Apple
ReversibleApple says that on iOS 9.3 and later the computer must be able to talk to Apple's servers, so test on a different network or machine.
Evidence ledger
Sources behind this answer
Exact questions
Common questions about “Apple iPhone or iPad code 53”
What does 53 mean on a Apple iPhone or iPad?+
Apple's article lists 53 with the errors that might mean a hardware issue, and links a separate Apple article for what to do about error 53.
Did Apple fix Error 53?+
Apple released an updated iOS 9.2.1 that restored phones disabled by the error and stopped non-Apple repairs to the home button triggering it.
Will restoring an iPhone clear Error 53?+
Apple says that if Touch ID was not working before, restoring will not make it work; the sensor needs service.
Will Error 53 brick my iPhone?+
In 2016 it did leave some phones unusable until Apple's iOS 9.2.1 fix. Phones on current iOS are not bricked by it, but a failing Touch ID sensor still needs service.
Which iPhones get error 53?+
The error is tied to devices with a Touch ID sensor, and it was mainly reported on iPhones with a replaced or damaged home button.
Can I just ignore Touch ID and use a passcode?+
You can, but the restore still fails while the sensor mismatch exists, so a repair is the real fix.
Can I fix error 53 by turning off Touch ID?+
No. The error is about the sensor not matching the device during the restore, so switching off Touch ID in Settings does not remove it; the sensor has to be serviced.