There was no problem with the iPhone, just want bigger iPhone memory, so the owner of iPhone 6 Plus expand the Nand flash via iPhone Nand flash programmer. After flash it, the iTunes report unknown error 4014. Typically, iTunes error 4014 is caused by NAND flash damaged. Now, we need to check it and fix it.
Put iPhone connect to computer via iPhone data cable, and then flash it with iTunes, it really reports unknown error 4014. It is restored and the serial number could be recognized, which appeares after Nand expansion. This is usually caused by the iPhone USB chips leading to iPhone CPU problem. We get to know it UMT motherboard after disassembly, then remove USB chip U1700 to measure resistance from solder pad to CPU, we find that AP_TO_TRISTAR_ACC_UART6_TXD shows the infinite value. And AP_BI_12C0_SDA is also broken.
The layer wire is disconnected, so we need to find out other connection point or digging for other holes, AP_TO_TRISTAR_ACC_UART6_TXD is direct connected without other external components, when digging the circuit AP_TO_TRISTAR_ACC_UART6_TXD we find that there is a hole next to socket, dig it to solder jumper wire.
AP_BI_12C0_SDA has a test point outside of the iPhone circuit board , welding directly from the test point. Then install the iPhone USB chip on it, and assemble the iPhone by phone screwdrivers.
Finally, flash it again with iTunes, now the iTunes progress bar can go smoothly and there is no iTunes error 4014 or other error code. The problem with this iPhone has been solved!