MacBook Air M1: Vertical black strip after screen replacement

I’ve just replaced a broken screen on my MacBook Air M1 and the new screen has a vertical black strip along the left hand side of the screen. I think it’s either a faulty screen or more likely, an issue with the cable. I’ve reseated the cable a few times but it hasn’t changed the issue. Is there a way to check if it’s the display or the cable? Any experience of how much a new cable would cost at an Apple approved vendor?