Default Proximity sensor issue

I'm sure my proximity sensor might be broken. Is there an app that forces the phone to stay awake until I hang up? Every time I make a call or receive a call the phone screen turns black and I am unable to wake it back up back up. My s4 is still under warranty but I dktn want to risk losing my mdk s4. Anyone know of an app that can force the phone to stay awake until I end the call?????

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