Apple Watch Series 9 showing VO2 level drop

Fix by:

1. Resetting fitness calibration
2. Doing 2–3 × 20-minute outdoor calibration walks/runs
3. Ensuring tight watch fit
4. Using Outdoor Walk/Run
5. Re-enabling HR & motion services

Your VO₂ max should return to normal within 1–2 weeks of corrected workouts.

My VO2 is decreasing since two weeks, no changes in my workout routine.

I use the default Outdoor Walk and Outdoor Run from time to time to check the VO2 estimate.

Two years ago it dropped significantly right after I entered a new age group after my birthday.

For me, this looks like a watchOS 26.1 issue.

Did you try to do the 1-Mile Walk fitness test, to find out roughly what your VO2 is, if you compare it to the same walk recored by your Apple Watch?

You can set a distance goal doing an Outdoor Walking Workout app and then compare the results your Apple Watch recorded with those of the 1-Mile Walk fitness test page, according to your age group.