Vertical lines are a common problem faced by DIY repairers after replacing the screen on an iPhone. Most of the time this isn't a defective screen and can be resolved with a little troubleshooting.
If the issue persists after troubleshooting using these steps you may have a defective part.
Check the display connections on the cables and the logic board. Dirt, dust or debris can cause a faulty connection and should be cleaned with a soft brush and high concentration isopropyl alcohol (at least 90%). Inspect the connectors and clean as necessary.
The display has an IC that could be damaged. Check particularly on the upper left and right corners. Any signs of damage indicate the part is defective and needs to be replaced with a new one.
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