I have two XTs. One failed last weekend on a ride. I stopped to fuel up, let the XT time out and shut down. When I started the bike it did not auto start up. The other one did. Since I was in the middle of a BBG run, I just used the other one to finish the run.
I tried to do the "soft reset" by holding the power button for about 30 seconds. That didn't work. I checked the other XT in the same cradle to make sure it would power it on when the key was cycled. Everything worked there. I suspected it was probably the battery. Since I have two XTs, I pulled the backs on both XT, and used the good one's battery to see if it would start the failed XT up. No go. Then I took the battery from the failed XT and used it to start the good XT. It came up, and indicated the battery was at 100%.
Then I called Garmin support and went through everything with them. Bottom line Garmin agreed it is an internal issue with the unit and had me package it up and send it in. They will send me a new one to replace it.
I bought both units in April 2021.
Just wanted to put this here to document the issue incase others have one. I learned there are no replacement batteries for the XT available on the market - found this out as I was anticipating the need for one.
Here is a video for taking the back off the XT if you should need to. The back has an oring style seal, and is easy to remove. One or two of the screws are shorter than the rest, so pay attention to were they go.