It seems like a few of us are having Dexcom issues lately! My neighbor came over today (she works for Dexcom & also has T1) to help me figure out what could be going on.
As far as the ??? on the receiver, she said that could be a few things. Often it could be because the transmitter is not snapped into the sensor all the way, and then when you shower or bathe, water gets in the way and produces the ???. I think this may have been our problem, as I've been having trouble snapping in the transmitter lately.
She also said that the ??? could be related to placement of the sensor and lack of oxygen and interstitial fluid. I think with our latest sensor error last night, it was due to placement, since the wire was bent a bit and there was some dried blood when I took it out. Our readings were WAY off, and I think that's why our sensor failed. She told me that when the sensor fails, and you think the placement is okay and everything, you CAN restart the same sensor and try again.
We've also had an issue once where the receiver re-initialized without warning, and she said that that is often due to static electricity (it's so dry here in AZ, so that makes sense.)
She helped me place a new one today (I just wanted to go over everything to make sure I was doing it right!) and I'm crossing my fingers for this one to last! I also called Dexcom customer service, and while they did ask me a lot of questions, they are sending a replacement sensor right away, so yay!