I've encountered error 53 upon transferring over a motherboard over to a original housing I used from another phone I bought from someone. I transferred many and never encountered this error prior. Only when the homeflex was in questionable condition has this happened to me on other phones. What's unusual is everything I used was in prestine condition including the charge port. home flex was also in mint shape. considering all i used was a clean motherboard that I put into this 6 Plus. Mind you the screen is also an original. Not an aftermarket replacement. This is very annoying because I've only been able to cure this issue one time using a smd reflow hose to reheat the chipset on an iPhone 6 homeflex which basically liquifies the solder mounting the chipset to the cables logic panel. This also does not fix the issue every time and the cable can only withstand so much of the process using the smd reflow solder hose. I've tried re-installation and replacing the flex cable that bridges from the homeflex to the motherboard connection under the LCD support panel it runs under. Still nothing. I believe it's not detecting it at all, because the phone will not enter DFU mode when using the button prompts for it. If home button was detected it would enter DFU mode. I will try to use another to just put the phone in DFU mode, then re-install the original back to see if the phone will restore properly.
Error 53 is considered next to impossible to repair, because so many variables can be influencing the Touch ID error. But if I can find a more solid solution that means more money can be made. Reflow soldering isn't always the solution even if the rest of the flexes involved are verified undamaged whether ripped, dented or twisted. In otherwords even if you have verified the parts are in place, with everything original, this problem is not for the faint of heart to trouble shoot and fix. Even pro repair shops don't bother once the customer has made it known to them that this is the error that shows up on iTunes.
I am seriously thinking of applying for a position at Apple after having a discussion about this with my co-owner just to see if I can find answers in their background.