iPhone 6 and error 4013?

Hi guys

I’m having a problem with an iPhone 6. It is stuck in recovery mode and I can’t get out of it. If I use a software to kick it out of the recovery mode, it gives a red screen in that case. What is the cause of this error? Apple’s website says it is the usb connection and on the other hand some people say it’s a bad part that’s been replaced. Other said it’s the NAND, sensor flex cable but I still didn’t figure it out.

What I’ve tried so far:

– I removed the whole screen and tried to restore like that, no luck
– I removed the battery, waited for a 30 minutes and put it back, no luck
– I removed the logic board, booted from the logic board connected to the usb port and battery only, no luck
– I checked the NAND chip with multimeter, it gave me the correct voltages.

It’s weird because sometimes very rarely I see the progress bar showing up and then it throws an error 9. But it happened only 2-3 times. 99% it throws 4013.

I don’t have any clues of what should I try and test more? If anyone has/had experience on this problem and solved it, please share your opinion and solution. I’ve googled a lot, but didn’t find nothing that helped me. That’s why I’m writing this post here. I need some expert advices on what should I do next.