Single vertical line on 27-inch 2012 iMac

I replaced the hard drive on my 2012 27-inch iMac with a SSD. Taking off and on the screen went smoothly. I've installed SSD's on over ten similar iMacs without any issues. After turning it on after the SSD was installed a gray single vertical line appeared on the left hand side of the screen.

I ended up purchasing a similar 2012 27-inch whose screen was broken but otherwise worked great. I took the screen from the original 2012 iMac and installed it on the newly purchased 2012 iMac with the same results. I used the same display data cable from the newly purchased iMac (which didn't have a vertical line when turned on with the broken screen). I the used the display data cable that was used in the original iMac with the same results.

Could the problem be with the original screen, the display power cable or something else? I'm puzzled.

Thank you for your help.