Samsung Galaxy S4 cannot connect to PC via USB after upgrade to Android 4.3 - Page 2 - Android Forum

I just spent HOURS on Google trying to figure this out. More than likely this is the answer, somehow your Qualcomm Modem settings got changed. If you are using Android 4.3 open the dial pad and dial *#0808# Android 4.2 *#7284# then under Qualcomm USB ...

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