You best source of information is the Apple support.
Dud you see this:
https://www.youtube.com/watch?v=zfUycPvFpjgA comment:
I have a bit of a problem. Basically, i have an HP L1908w monitor and i connect a VGA to that one one end, on the other end there is an HDMI cable with connects to my thunderbolt adapter thing. But when i connect it, it doesn't show the screen. The monitor works but it only says Check video cable but nothing is wrong with it. Any help?
So some people are having trouble.