Bootloop with only battery connected to motherboard
Hello. I have a Pixel 3a I gave to my son several months ago. I've worked on it several times recently, after the power switch got mushy and I thought it would be good to replace the battery. I've taken it apart several times, no issues ... until now.
Just yesterday I took it apart to replace the power switch and battery, only to find that the brand new, OEM battery I'd bought was bad (puffed up).
In the process, I somehow broke the fingerprint sensor cable, so now that's no longer attached to the motherboard.
After putting it all back together, I found I was stuck in an infinite bootloop (gets to the logo for maybe half a second, every other loop). Can't get into any rescue function. E.g., I can send a fastboot boot for TWRP, and it's in that state long enough to send (maybe 2 seconds), but then just bootloops again. I can't change the fastboot menu, either.
The most important thing is that it's like this even when only the battery (and screen) is connected to the motherboard - nothing else. It's always on, and always bootlooping.
I've confirmed the problem isn't with any attached part, including the battery or power switch (I've tried both my old and new parts in each case), and again, it's like this when it's just the bare motherboard with the battery attached.
Am I SOL, or does anyone have any suggestions for trying to resuscitate. Could having broken the fingerprint sensor have any role in this? Or, more to the point, does anything need to be connected to the motherboard other than the battery for it to work correctly?
Thanks for any ideas.