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Common causes of this problem include iPhone software problems, low battery power of AirPods, dirt on the AirPods case, and the iOS system has not been updated, etc. If it is an iOS system issue, a useful repair tool such as TunesKit iOS System Recovery is very important for me.
 
I've successfully fixed an airpod that stopped working and none of the standard troubleshooting steps could solve. I aggressively cleaned it with 99% IPA using a squirt bottle/syringe and q-tips. I soaked the small grating holes on the back and scrubbed them until I couldn't see any debris under a microscope. Then I let them dry for an 10 minutes and it works flawlessly. No impact to sound quality (I'm not sure had I used 70% IPA, it would have worked).
 
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