Our QC-35 power switch stopped working after a while. Specifically, it was no longer able to slide over to begin pairing or to select a new device.
The cause was a crack in the plastic switch. The crack was only visible after the headphones were disassembled. The purpose of this bracket is to wrap around the plastic switch and hold it together. It also stabalizes the switch and prevents further damage.
Once installed, this bracket restored the function of our switch. The switch now slides easier and feels more secure than it did when new. The bracket is fully internal and can not be seen after the headphones are assembled.
Your results may vary, but if the switch is broken, it might be worth a shot. Open the headphones using this guide:
https://www.reddit.com/r/bose/comments/5vb4u4/bose_qc35_disassembly/dfw2c7c/
Update: A
complete replacement for the switch is also available.