What proximity sensor does is that it tries to recognize - yet not always right - when you hold your phone next to your ear, so that the phone will lock (turns on the black screen), then you won't accidentally press keys with your cheek when talking to other people on the phone. Once you're done with your conversation & remove the phone from your ear, the sensor is then supposed to unlock your screen & allow you to end the call.
This makes sense in theory, however in my case I had my phone laying flat on the desk, nowhere close to my ear. Or holding in my hand but had it on a speaker. Still it locks (black screen). Very inconvenient feature if you ask me, as I had a great difficulty activating my new credit card while unable to enter the numbers. I wish there was a way to disable this. Argh LG.
I ran a proximity test on the phone. Was holding my hand over the top of the display while making a call, and then the screen locks. Remove hand and it unlocks. This means that the proximity sensor is working. So far so good, now am searching high & low as to how to disable said sensor or change the distance. Very frustrating.
A temporary solution to hold my hand over the screen while it locks helps to unlock. It however locks up again, then I repeat Step 1. Oh well.