I used to click the back arrow every time I saw the OTA update to Android 4.3 message, to not upgrade. I accidentally hit the Start (right above the back arrow) yesterday :(
So, I'm at Android 4.3 now on my S4, and my S4 's ability to use my car's audio system to handle phonecalls via Blutooth no longer works reliably. The phone is still paired with the car, and I saw it kinda go in and out of bluetooth connectivity, once. Is the new low power Bluetooth in Android 4.3 causing this?