Connect the iPhone 6S Plus to the computer, and then use iTunes to restore it, the iTunes reports unknown error 4013, and the UDID information can be displayed. As we know, for the iPhone 6S Plus automatically restarts and iTunes 4013 error problems, basically it's the iPhone Nand flash problem, but this iPhone can display UDID information, so, the probability of the iPhone Nand flash failure is relatively small.
Now we need to disassemble the iPhone 6S Plus. Use phone opening tools to disassemble the iPhone 6S Plus, remove the iPhone display assembly, disconnect the iPhone battery, and then remove the iPhone motherboard.
Observing the iPhone motherboard, we can see that the iPhone CPU, iPhone Nand flash, and iPhone USB chip all have all been welded.
First we need to clean up residual solder paste, and then put iPhone motherboard under the Binocular Stereo Microscope to see if any components have fallen out of the welded place. After observation, we confirmed that all components are there.
According to experience, the iPhone baseband power chip also will cause this problem. Open the ZXW Dongle to check iPhone schematic diagram and bitmap, we know that the iPhone 6S Plus baseband chip communicates with the CPU by using the PCIE bus, the PCIE bus needs to be initialized during boot.
Now restore the faulty motherboard, and measure the power supply for the iPhone 6S Plus baseband when the Apple logo appears, it's found that the power supply of the PP_OV9_SMPS1 and PP_1V0_SMPS3 has been generated, but the PP_1V225_SMPS2 and PP_1V85_SMPS4 doesn't generate, which means the iPhone baseband has no problem, so it could be the iPhone base with empty welding or damage.
Remove the iPhone baseband IC, and then use digital multimeter to measure the diode data for the iPhone baseband eeprom IC and iPhone CPU, the data measured all are normal.
Reballing the iPhone baseband IC, and then use Quick 861DW hot air rework station to solder the iPhone baseband IC back the motherboard. Restore it again, now everything goes well.
Hope our repair method will help you. If you have other better opinions, please leave a message in the comments.