Vertical coloured lines on the monitor
Can I start to the iMac with an external monitor? Note that I cannot see anything on the monitor other that the lines.
iMac 27″, macOS 10.13
Question marked as
This sounds like a failed GPU, while you may be able to use an external display if the GPU is failed as I believe it is then you will have the same issue with the external display. My recommendation is to take the computer to your local Apple Authorized Service Provider to have it professionally diagnosed and then you can explore your options. If you don't know of any AASP's in your area please click the following link to begin locating one.
Apple Authorized Service Provider Locator (AASP) & Genius Appointment
Good luck!