So, not sure if anybody has commented on this fix before, but here goes.
A couple of weeks ago when my sensor went ??? on me after 8 days, after stopping it I looked under the transmitter, after I pulled the sensor out, and found that a lot of moisture had gathered near the contact points. I made a mental note to check again the next time I saw ???.
A few days ago, again at around 8 days of sensor life, I got the ???. I checked the transmitter for moisture at the contact points, before trying to replace the sensor this time. Sure enough, there it was, lots of moisture. I wiped off the water, clicked the transmitter in, and voila, ??? gone. This morning, same thing. I got ???, removed the trasmitter while keeping the same sensor in, and found more water. I cleaned the transmitter off again and everything is working fine again.
So I havea new SOP for when i get the pesky ??? that doesn't want to seem to resolve itself. I don't know why I keep getting water underneath the transmitter repeatedly, but one solution at a time.