Q: why do I have a yellow vertical line down the middle of my screen?

Greetings steinbis,

Welcome to the Apple Support Communities!

I understand that you are seeing a faint yellow vertical line down the front of your iMac’s display. In this situation, I would recommend reading over the attached article and working through the suggested troubleshooting steps within such as resetting the system and resolution. 

Apple computers: Troubleshooting issues with video on internal or external displays - Apple Support

Cheers,

Joe