In a new support document sent to retail stores and licensed service providers this weekend and obtained by 9to5Mac Apple described an updated repair process for customers Having Problems with Face ID the iPhone X

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Essentially, the paper explains that employees should first run a diagnostic test to see if the problems are getting worse. Facial identificati of the customer can be solved with a rear camera repair. If this does not happen, the employee must simply perform a full replacement, giving the customer a new phe.
Although repairing the rear camera to correct facial identificati problems may seem counter-intuitive, the first time we've heard about it. For example, shortly after the launch of the iPhone X, a user asked Reddit to explain that the rear view camera and face recogniti had stopped working. This user noted that features like pano, slow-mo and time-lapse still worked, but everything else was not working. In additi, when the pers would try to use Face ID, she would see a message saying "Face ID is not available, try again later."
Eventually, this iPhone X user had his device replaced by an Apple Store, with the An employee of Genius Bar explains that the True Depth camera and the telephoto lens are cnected:
I was talking to the guy at Genius Bar and he said …