Welcome to Justanswer.com, my name is Dustin.I'm sorry to hear about the issue you are having, I will do my best to help you resolve it.
Usually, we just need to reset the devices internal software to solve issues like this. Heres how to do that
To perform the master reset:
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.
Once complete, test the device.. If its still not reading ri ght, let me know and we can try other steps as needed.
I strive to provide great service, and greatly appreciate a 5 star rating clicked and submitted when you are satisfied.. I always answer follow up questions.Dustin S.Senior Technician and Network Switching Systems Operator / Maintainer.
You will.. But thats the only way to "right" the software.
You could use garmin connect or express to backup your data first..
Understood.. It might be a good idea to connect the device to your computer, and then update the software through garmin express as well, before your ride.