I had the same problem, traced it to a bad contact to left side speaker itself, inside of its enclosure. Pressing a little on the speaker membrane re-established the contact. As the enclosure is sealed/glued, I couldn’t get inside to resolder it. Bought in 2015, so this may be part of the defect series mentioned by another user.
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(Furthermore, the coil wire seems glued to the membrane so with sound vibrations it’s entirely possible to cause fatigue in the small conductor, so I don’t know if I’d have been able to fix it even if the speaker enclosure wasn’t glued in, because the membrane is plastic so it would have reacted poorly to a soldering iron.)
I had the same problem, traced it to a bad contact to left side speaker itself, inside of its enclosure. Pressing a little on the speaker membrane re-established the contact. As the enclosure is sealed/glued, I couldn’t get inside to resolder it. Bought in 2015, so this may be part of the defect series mentioned by another user.
I’ll order another ANC headset but not Bose.