If it is the Thunderbolt Display, it's possible that there's something wrong with the computer when it's outputting the Displayport signal, although that's unusual seeing that the MDP->VGA adapter works. You might as well get an active one as it's only a few dollars more and it will work with higher resolution monitors if you need to do so in the future. If it is the Cinema Display, you need to try a different MDP-HDMI adapter. So check the System Preferences -> Display. WAVLINK USB C to HDMI VGA Adapter 2 in 1 USB 3.1 to 4K HDMI + 1080P VGA Adapter for MacBook Pro/MacBook Air, iPad Pro, Dell,HP,Samsung Galaxy,Huawei Mate 30/P30. Either of the later Cinema Displays or the Thunderbolt Display will connect to the same port on the MBP, but the signal they use is different. If you connected to an Apple Thunderbolt Display - that uses a Thunderbolt signal from your computer, which is not the same as the signal used when connected to an Apple Cinema Display. Match the shape on the end of your cable to the HDMI, Mini DisplayPort, or USB-C port on your MacBook and connect it. 3 Connect one end of the cable to your MacBook.

So this is important - if it is an Apple Cinema Display, it would have be the one with the MiniDisplayport input, in which case, your computer is fine and it's outputting the correct signal, it's likely the MiniDisplayport-to-HDMI adapter that's the issue. The kit contains 3 adapter products to connect your MacBook Air or similarly configured MacBook Pro to an external HDMI or VGA display, and a. If you're using a MacBook with USB-C ports, you'll need a USB-C VGA Multiport Adapter for VGA displays or a USB-C Digital AV Multiport Adapter for HDMI displays.

Macbook air vga hdmi adapter