I'm attempting to set up my runcam link falcon nano. When connected to my speedybee f405 v3 and powered with a lipo, a faint red light appears on the runcam link, which doesn't change color. Surprisingly, when I tested it with another speedybee f405 v3 flight controller, it worked perfectly, pairing with my DJI goggles. However, when I tried using it with the flight controller I want to use, it remained stuck on the red light. I checked that MSP is enabled on the correct UART and replicated the exact settings from the flight controller it worked on, but the issue persisted. I even tried soldering the air unit directly to the flight controller, suspecting a port problem, but the problem remains. I have tried researching and attempting various solutions I found online but I haven't been able to make it work.

Here is what I am using:
Flight controller: SpeedyBee F405 V3 30x30 Flight Controller (with Firmware 4.4.2)
ESC: SpeedyBee F405 BLS 50A 30x30 4-in-1 ESC
Receiver: TBS crossfire nano
Air unit: Runcam Link Falcon Nano HD FPV Camera Kit for DJI HD FPV (HD-FPV-LINKFAL)
Goggles: DJI FPV goggles V1
Battery: CNHL Speedy Pizza 1200mAh 22.2V 6S 100C LiPo Battery
Radio: FrSky X9D taranis with TBS Crossfire module

I would blame the FC. I had a similar issue with a Skystars FC. The vista would not work no matter what I tried. I moved it to another FC and it worked flawlessly. I've not bought any more Skystars products.

I've also had a number of issues early on with the Speedybee F405 stack. It would just stop working. Next one did the same thing. The third one has been working good since I put it in months ago.

I firmly believe that Gremlins are being included at no charge to us.

But as an obvious suggestion; did you try a different UART on the suspect board?

The LED on the Air Unit Lite should turn from red to green after about 15-20 seconds even if it's powered up without being connected to a flight controller. The LED status turning green means that the Air Unit is successfully communicating with the camera. If the LED remains red, in most cases it means there is a bad connection between the camera and the Air Unit, or you have a faulty camera. Carefully try re-seating the MIPI connectors on each end of the camera cable by gently unplugging and re-plugging them. Maybe you have a dodgy connection and you disturbed something when moving the Air Unit between different flight controllers.