The dropped iPhone 6 reboots itself after displaying white Apple logo and it reports unknown error (4005) when rooting.
Assemble board on testing assembly to root. Turn in on and connect it to computer. The i4 assistance says that iPhone is getting into DFU mode. After it is rooted, it reboots. Then it standbys until it reports unknown error (4005).
Based on my experience, faulty NAND FLASH, CPU upper board and disconnected motherboard would cause such problems. Then I begin to fix them. First measure the diode data to ground of chip. The data is normal. After tearing the NAND FLASH, measure the diode data to ground that NAND FLASH use it to communicate with CPU. The data is normal too. Replace the NAND FLASH and root. The problem is still the same.
Then solder CPU again to root. The problem is still the same. Tear the NAND FLASH again and measure its diode data to ground. I find that a solder pad is pseudo soldering. Using diode pole of multimeter to measure, if I press hard, the probe will show resistance data. The pseudo soldering pins are shown in picture 7-49.
If I press hard, the probe will show resistance data. If I press slightly, the probe will not show, which suggests that the board is faulty. I have to find the pins that connect CPU, as shown in picture 7-50.
Jump a wire from CPU solder pad to the solder pad of NAND FLASH, as shown in picture 7-51 and 52.
After jumping the wire, assemble CPU and put motherboard on assembly to root. It now shows progress bar, as shown in picture 7-53. When root finishes, it can get into IOS. After testing, all the functions are normal. Trouble is removed.