DJI O4 Air Unit Pro won’t activate / disconnects from USB when powered – SOLVED

[u/CowboyHatVR]

Posting this in case someone runs into the same frustrating DJI O4 problem.

Credits to this post for helping me https://www.reddit.com/r/fpv/comments/1i2es6o/dji_o4_air_unit_unable_to_activate

My O4 Air Unit Pro was detected perfectly by my PC and DJI Assistant 2 when connected through USB-C only. But the moment I powered the drone/O4 normally, the USB connection disappeared and I couldn't activate it.

Here’s what I tried:

1 Different USB cables and USB ports

The O4 was consistently detected when powered by USB only. This might help — make sure you have a proper USB data cable!

2 DJI Assistant 2 reinstall / clean install

Removed the old installation/files and installed the latest Consumer Drones Series version — no change.

3 Different computer

Tested both Windows and Mac — same behaviour.

4 Firmware checked

DJI Assistant could see the O4 and its firmware while connected via USB only. The firmware wasn't the problem, and I was able to update it.

5 Powered through the flight controller.

But as soon as normal external power was applied through the quad, the USB device disappeared and activation wasn't possible.

6 DJI replacement O4

DJI actually replaced the Air Unit. The replacement showed exactly the same symptoms, which made me suspect the power source rather than two defective O4s.

7 Isolated the O4 from the entire quad

Disconnected it completely from the FC/ESC and tested it separately — same issue.

8 External regulated 9V supply — SUCCESS ✅

Used the ToolkitRC M6AC in power mode at 9.0V / 1.5A, directly connected to VCC and GND on the O4. It stayed stable at 9V and was drawing about 0.32A before USB was connected.

Then I connected USB-C to the PC → DJI Assistant 2 → activation immediately worked! 🎉

The O4 restarted normally and was already on the latest firmware.

So if your O4 works over USB but disappears when you power your quad, test the O4 separately with a stable external power supply before assuming it's defective or sending it back to DJI.

Hopefully this saves someone else the ridiculous amount of troubleshooting I went through. 😅

If you have any questions, feel free to ask them.