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We may have to reset the device. And we may need to connect it to a computer and update the software.
Lets try the reset first.. Heres how~
The reset on the Forerunner 910XT is successful if the device finishes powering up and goes to the setup wizard. Try the steps again if the setup wizard does not appear.
Leave the device outside with a clear view of the sky for a minimum of 20 minutes to acquire satellite data once the reset and setup wizard are complete.
Let me know if that fixes the issue and if not, we can try other steps as needed.
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