Error 53 is related to hardware integrity issues. It means that if your iPhone 6 or 6Plus suffers a damage that which affects the Touch ID or if you get it repaired from a third party then the device is unable to complete the security checks.
A diagnostic check inspects the Touch ID hardware before the phone is shipped from the factories and if the phones fails validation you will get Error 53. Some users also found this error when they were updating their device to the latest iOS. The phone would get stuck in Recovery Mode and if you try to restore the device through iTunes, you will find this error message on the screen.
Solution: Update the phone software
Apple released iOS 9.2.1 patch through iTunes to fix the problem. The patch was released on February 18, 2016.
- Visit the Settings menu of the Phone
- Tap on General
- Tap on Software Update
If you notice a new version available then install it.