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.
When you reset the Edge 800.. Whats the exact method you used?
Lets try specifically, a master reset
To perform the master reset:
The Edge 800 is now reset. Leave the device outside with a clear view of the sky for a minimum of 20 minutes to acquire satellite data.
Once complete, connect the edge to your computer and download and install the update released on november 14th 2016
Once you h ave downloaded the newest version of garmin express and allowed it to update the edge 800.. Test it out again on your next ride. Let me know if its working correct ( in regards ***** ***** cadence sensor ) or not.
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Hello again, I havent heard from you for a while. Did you still need help with this issue or not?If you do not need help, please remember to click and submit a 5 star rating to show that I was able to answer your questions. If you still need help, please send a reply so we can continue working on the issue.Thanks again,Dustin S.