http://nhv.us/content/16024213-apple-faces-customer-backlash-error-53-renders-iphone-6-useless
Apparently, if you had your iPhone 6 repaired by someone other than an Apple service technician or Apple authorized repair center, upgrading to iOS9 could permanently lock and disable your phone with Error 53. There's no reset or downgrade that will fix it. Apple knew about Error 53 before iOS9 was released, but they didn't tell users who now have to go to Apple for out of warranty repairs.
Error 53 is caused when iOS9 can't validate the Touch ID module on the phone matches everything else on the phone. It could be caused by Touch ID module replacement or screen repairs.
Apparently, if you had your iPhone 6 repaired by someone other than an Apple service technician or Apple authorized repair center, upgrading to iOS9 could permanently lock and disable your phone with Error 53. There's no reset or downgrade that will fix it. Apple knew about Error 53 before iOS9 was released, but they didn't tell users who now have to go to Apple for out of warranty repairs.
Error 53 is caused when iOS9 can't validate the Touch ID module on the phone matches everything else on the phone. It could be caused by Touch ID module replacement or screen repairs.