I had an issue with my Pixhawk 4. On powering up via USB, the FMU power LED would flash red at very high frequency for a few seconds, then it would stop, and I'd have no FMU lights on at all. On the other hand, when powering up from the battery, the same LED wouldn't even flash upon power up.
From reading around, I thought that the FMU may be in boot mode. I wasn't quite sure what was wrong, and neither was my supplier when I called them, so they agreed to change it and I sent it back. But I'm still interested in what the problem was, primarily so that it doesn't happen again.
I was just wondering if anyone might be able to guess at what the issue was based on my description?
In case anyone was wondering, the device has never been on a flight, and we've just been configuring the setup, so it certainly didn't take a hard knock. We flashed Ardupilot onto the board a couple of weeks ago, and everything was working fine up until now.
Other than a component on the board just being faulty, the only other thing I can think of that may have done something weird is that we were trying to interface with the board via blheli_32 passthrough to tune the ESCs. This software is Windows only, and we were running it through Wine on a Macbook. I wondered if the cross platform translation of the serial port communication may have been a bit confused and caused the FMU to fail (I'm aware that software doesn't always run perfectly through Wine). I think this is quite unlikely, but it's the only slightly non-standard thing I can think of that we've done with it.
Any input welcome.