4BCC - DDE: Reducing Agent Passive-tank fill-level sensor signal

[Best4x4xFAR]

So one of the few 'Present' Codes right now per the Foxwell Scan is this 4BCC. I've found multiple threads about the code, and the associated 'full' sensor on the passive DEF tank. I've cleared it and it comes right back.

Seems it is fairly common for it go bad, and I've also seen 'fixes' that include a VW sensor replacement (though I'm not clear on the details), and the 10 Ohm resistor across the contacts that essentially tells the system the tank is always full I would assume (no?)..

It seems clear that the sensor is not available as a replacement part, and that BMW insists you buy the whole tank for replacement.

Not clear what the function of this sensor is, and what the side effect is to the systems to basically bypass it with the 10 ohm resistor trick. So if anyone knows and can fill me in, that'd be great..

Also, anyone know from experience if this is covered under the 10 year/120k CARB states extended emissions warranty?