My iPhone 4 power button stopped working, it would not click down. iOS 6 beta 2 had bricked my iPhone and I had no way to perform a hard reboot (i.e. - holding down power and home button). After doing some reading online I decided it had to be the power and sensor cable. This was confirmed after disassembling my iPhone. The cable was missing the small black piece of plastic that sits on top of the orange sensor. Tiny!
The repair went great! I successfully installed my new power and sensor cable. The power button is now working and I was able to get the phone to turn back on.
The guide doesn't mention the braket or small foam ring for the sensor that is attached to the original power and sensor band. The replacement piece I received did not have this. Be sure to transfer them to your new band!