Is the blue LED next to D7 on dimly? That usually indicates a corrupted flash and the bootloader cannot be run.
The charge LED is driven by the PMIC, so it’s at least partly working and the USB port has power.
If you have a voltmeter, you can check the voltage on 3V3. Also check pin D7.
To make sure the firmware isn’t putting the device asleep really quickly, hold down MODE and tap RESET. That’s probably not the case because the status LED usually blinks white briefly before going back to sleep.
If you haven’t done so, disconnect both the battery and USB power and let the device sit for 30 seconds so it’s completely discharged before plugging in the USB power. With USB powered but the battery disconnected the charge LED should flicker.
I guess it’s probably the 3.3V regulator (TPS62291), not the PMIC (bq24195), but it could be either. Running the battery down won’t affect either of those chips, but it could cause the Dim D7 issue, which is not the problem you are seeing. In any case, it’s most likely a hardware failure that’s not easily fixable, unfortunately.
I already ordered a PCB with this on it.
I guess I could try replacing those parts - but I could (and probably would) damage something else in the process.
Is there a way I can mail it in for a replacement?
I probed some test points and the 3V3 is measuring 0.3V. I scoped the output of the IC (see below) and I think it looks ok. Then I thought it could be the 10uF cap so I checked that and it is good - so that leads me to think it could be a problem with the inductor. I tried to check its resistance to see if there’s any issues there but I couldn’t really get to the pads. I might need to lift it off the PCB to test it - I’ll try that when I get more time to work on it.
If you have a 3.3V bench supply, try briefly powering the device via the 3V3 pin. This is not recommended as it can damage the regulator, however there’s a good chance the regulator is already damaged. If the status LED turns on (probably white to start), then that is a good indication that the STM32 is still OK. Don’t leave it powered for long, and the device will not fully start up without the 3V8 power supply anyway.