[AC Question]

With the screen flashing on and off I can't access the keypad to enter anything or end a call. When I shine my flashlight on the proximity sensor it works right.

[VidJunky]

You've hit the preverbal nail on the head with the flashlight thing. Chance are your sensor is dirty, it may not look too bad but these sensors are sensitive and sometimes it only takes a little bit of grim, from makeup, sweat, or your pocket to cause it to act up. Do you have a screen protector on the device that also covers the sensor? Screen protectors are natorious for turning colors, brown, yellow, etc., and just being able to see through the protector doesn't garantee that it can see you back. If you have a screen protector that you can remove just the bit that covers the sensor try that, clean the face of the phone up real well including the sensor, or remove the cover your screen protector is attached to and see if that doesn't put function back into your device. I will also add that these sensors don't usually go bad, but when they do usually fail on or off, so they either think they are seeing light when they are not or they never see the light, I think I know a few people like that, but anyway by shining a light at it and it working shows that something is blocking the light dark signal and the sensor is working. If you are thinking of an app to check the sensor there are some, as well as codes you can put into the device to activate a self diagnostic, but if your sensor is dirty or otherwise inhibited they won't really diagnose the issue unless you've done the minimum.

[schemist]

I was able to look at the sensor under a microscope at work and saw it was dirty under the screen on the inside. Spraying compressed air into the speaker by the sensor cleared away what was interfering with the sensor. Works great now.