Yamaha TW-E3C True Wireless Earbuds (Red) - Manual (Page 46)

Sound can only be heard from one earbud
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Earbuds (L) and (R) are not connecting properly.
Put the earbuds back into the charging case and then take them out again.
The earbuds turn on and both earbuds automatically connect after the earbud indicators light up for one
second. If successfully connected, you will hear a sound effect.
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The signal may not be coming in clearly, making the connection unstable.
Move away from wireless devices such as access points, or appliances that produce electromagnetic
waves such as microwave ovens. After placing both earbuds into the charging case, take them out again.
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You may be using a smartphone app that generates sound from only one channel.
Check whether you can hear sound coming from both left and right channels by playing back sound from
a different app.
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One of the earbuds might be turned off.
Turn the power on.
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“Turning on the power” (p.16)
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The connection information for the registered Bluetooth device may not be working properly.
Delete the connection information of this product from the Bluetooth device, and then pair the earbuds
with the Bluetooth device.
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“Pairing with a Different Device” (p.21)
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Troubleshooting > Sound can only be heard from one earbud