Left AirPod won’t work
My left AirPod won’t connect even though it’s charged and full
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Did you already try the troubleshooting explained in this related article?
If your left or right AirPod isn't working - Apple Support
If these steps do not work, this looks like a hardware problem and you might have to replace the left one.
AirPods and AirPods Case Replacement - Official Apple Support