So, you had problems with your iPhone 6 or later device, took it to your neighborhood technician, and she replaced the Touch ID sensor with a new one. Everything worked fine until you updated to iOS 9 or later, at which point you began seeing a mysterious "Error 53" message. Apple today acknowledged that it's not aliens infesting your iPhone, but a problem brought on by having an unauthorized technician fix your device.
According to a posting in The Guardian, an Apple spokeswoman explained that "We protect fingerprint data using a Secure Enclave, which is uniquely paired to the Touch ID sensor. When an iPhone is services by an authorised Apple service provider or Apple retail store for changes that affect the Touch ID sensor, the pairing is re-validated."
However, when an unofficial repair that pairing can go unvalidated and cause the Error 53 message to appear when iOS is updated or restored on the device. The spokesperson suggested that people experiencing the problem should contact Apple support. But this may end up causing affected owners to buy a new iPhone, since unofficial repairs violate the terms of the iPhone warranty.
The answer? If you have an issue with an iPhone and you can't take it to an Apple Store, be sure that you find an authorized Apple service provider to fix it. If you can't find one, send it to Apple or else you may end up buying a new iPhone.