Device boots, however, the screen is blank and no vibrations.

Hello ifixit members,

I have an iPhone 6 Plus with some problems. Although the devices still boots and charges, the screen stays blank and the vibration motor seems to be dead.

1. On the 8th of October, 2016, I swapped Apple's deteriorating battery with a fresh one from ifixit.
2. On the 27th of November 2016, I drop my 6 Plus, face first, on a concrete floor, cracking the screen.
3. I order a replacement screen from ifixit and install it. Replacement screen is bad so I cross-swap it for a good one.
4. I install the new replacement on the 8th of December 2016. Phone is as new.
5. On the 9th of January, I push the home button of the device to check the time. The device's screen displays an artefact for a second, then turns black but not off. I do a hard reboot but nothing gives. The device is 'dead'.

• That same night, I open up the device to check if any of the display assebly cables came loose from their socket. I find two screws from the screen assembly, in my phone. It seemed they were not properly secured to the replacement display assembly. Take note: I never had to touch these screws during the replacement process!
• As I take a look at the display cables on the guard everything seems to fit nicely. I disconnect all display cables and reseat them. I close the device, and do a hard rebout. No effect.
• In coconutBattery, no device information is displayed, yet iTunes seems to detect the iPhone and asks me to unlock device. Since the screen does not react I can't.
• I put the phone in DFU and did a software reset. coconutBattery displays device information and charges the phone.
• Whenever I connect the phone to the computer, it does not vibrate.
• I was curious and decided to do some tests. I opened the device again and connected a spare battery. The screen turns on but is unresponsive to touch. The device also vibrates like crazy.
• I reinstall the ifixit battery. No dice.
• I try the spare battery again. No dice.
• I reinstall the ifixit battery because the problem does seem to be electrical but not necessarily related to the battery.

Ever since I have done multiple DFU-restores, power cycles, reseating of the relevant cables etc. ...

I don't know what to do. At first I thought I had the dreaded U2 controller chip problem. That does not seems to be the case as the phone charges properly. My main two suspects:

1. The logic board and, more specifically, the display connectors themselves or an electrical defect. (Although they look fine when I checked the with my jewellers loupe.)
2. The display assembly, maybe it just died... I don't have the money to just buy another one and test that :(.

EDIT: The software aspect is also a bit strange... I can restore the phone to a limited degree due to the ability to DFU the device. However, when I remove the activation lock and insert a SIM, the phone does not detect the SIM. Another point for a possible logic board problem.

Can somebody help on this road to a solution?

Kind regards!

Fabio