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I replaced a iPhone 6 battery yesterday (06/02/18) and since could not get the side power button to work, but the main problem is that since then, the screen had grey bars on it. I disconnected the screen cables and reattached them in the right order/slot etc. Now I have these bars showing:

Please help as I don’t want to have to buy a new phone

It’s a bad replacement lcd more than likely. Try another screen, if you have one handy.

does the touch work ?? try bending the phone check if your battery is pressed against the screen i too have some bars appearing on my phones screen after it fell off my bed right know i am bending my phone a bit to make the bars go away but it comes back after some time check if your phone has problems with your ic-chip