Grey lines even after screen replacement

Hi! I have some trouble with an iPhone 5. The display of the phone shows those standard grey vertical lines that you can get after a screen replacement. The special thing about this phone is that the lines are there on every single display I have tested. I've tested 4 unoriginal and 1 genuine display on the iPhone, and the lines are still there. I guess that there's something wrong with the motherboard, but do you have any solutions? Do anyone know which part on the motherboard which eventually should be replaced?