Bose Quietcomfort won't turn on - open it and check if a plastic piece sitting on the switch is intact.
Bose Quietcomfort won't enter pairing mode - it's possible that the switch on the PCB can't reach position responsible for entering pairing mode. Open the headphone and check if moving the switch directly helps, if so, continue reading.
Headphone won't turn on when the switch is moved.
The problem was this small plastic part which is visible from outside as a switch and which moves the actual switch soldered on the PCB. To fix it you will have to open the right headphone. Start with gently removing the ear cushion and the fabric inside, then undo three screws:
Without screws aluminium cover can be removed. The actual switch is highlighted:
From the other side of the aluminium cover you see a plastic part which should sit firmly on the switch. In this case it is broken:
Find two pieces of the broken part and glue it together, I used two compound epoxy glue.
It's not pretty but it holds well.
Bose Quietcomfort won't enter pairing mode.
The mechanical tolerances of the switch, this plastic piece and aluminium cover are so tight, that it can happen that the headphone will not enter pairing mode. You move the switch but it neither enters pairing mode nor removes paired devices. The reason is, that the oval hole in the aluminium cover is to small and the switch will not contact in the "bluetooth" position. I extended this hole. It was around 8,9mm long but I extended it by approximately 0,75mm towards the bluetooth logo. I used a round file for this:
It is barely visible that the hole is longer:
But this bigger hole allows reaching the final position of the switch.
