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...
Take the cartridges out & clean the contacts with Q tips. Have your Q tip damp but not over-soaked with water. Then dry the cartridges with the dry end of a Q tip and then with a soft cloth. I used tap water, and after drying went over with a hair dryer just to make sure. It worked.
I'm thinking that a battery door might be making intermittent contact with the batteries, on and off. I have a similar problem with my Canon A480. This is pretty common to most if not all, compact cameras. The solution is to repair or replace the battery door.
This is very common for the wireless HP printers. My HP Deskjet 3630 does the same. The way around this is to take a screenshot of the text and paste this into your word-processing program. From there, you print the image.