I don't see a start button to measure blood oxygen level
I don't see start button to measure blood oxygen on my apple watch
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If you followed all the steps in the 'How to take a blood oxygen measurement' section of:
How to use the Blood Oxygen app on Apple Watch - Apple Support
then 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