Apple watch series 3
Why is my apple watch series 3 showing 2 green lights at the back
Apple Watch
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Those are the heart rate sensors. See the diagram in the 'How Apple Watch measures your heart rate' section of:
Monitor your heart rate with Apple Watch - Apple Support
BTW, they should remain on constantly (except maybe during a workout) or it will drain your battery quickly. If they stay on all the time, the first (and simplest) thing to try if something isn’t working right is to restart your Apple Watch and its paired iPhone. If necessary (device not responding), try a forced restart. Both methods can be found here:
How to restart your Apple Watch - Apple Support
Restart your iPhone - Apple Support
If that doesn't help, the next steps would be to unpair and pair your watch again and restore from the backup just created. This should result in no loss of data. It deletes all temp and corrupted files and re-indexes the file system.
Unpair and erase your Apple Watch - Apple Support
Set up your Apple Watch - Apple Support