Hopefully, when someone like me searches for answers as to why a 2015 MacBook Air won’t output more than 2 video resolutions through their Thunderbolt to VGA adaptor, they’ll find this post and won’t waste as much time as I have.
(** see this post on the same subject with additional details **)
Long story short, if you’re trying to use an adaptor that’s not Apple branded, you’ll only get 1280×1024 or 800×600 resolution choices from a 2015 MacBook air with OS X 10.10. For example, we purchased a NewerTech Mini DisplayPort to VGA Adapter from Mac Sales (NWTCBLMDPVGA) and had no trouble with it on MacBook Pros (2011 +) but the 2015 Macbook Air wouldn’t provide a higher resolution.
There are identical issues with the Amazon Basics Mini Display adaptor. (Click link for a review with a screen shot)
After resetting the SMC, “zapping” the PRAM, running in SafeMode, etc., etc., it seems to be that something has changed with the 2015 display port and third party adaptors now have limited functionality. I can’t find anything specific that says just that, but it’s much cheaper and safer to drop the $29.99 at Best Buy for the adapter than to try any kind of firmware update or hack.
It’s interesting to note that the third party Mini DisplayPort to HDMI Adapter works as expected.
This adapter is from the same manufacturer as the VGA adaptor (NewerTech).
So, stop trouble shooting cables, connectors, adaptors, the OS, and system preferences.
Run out and grab an Apple branded adaptor and get back to it.