My proximity sensor went bad after I replaced a screen lens by heating up and unglue technique. I turned it off in the phone app, and now it's fine, but for me it's very irritating that I have a high-end device which is somewhat broken, so I started researching what's wrong and how to fix.
It does work, I tested it by enabling hidden menu ( *#22745927 ) and then diagnostic mode ( *#0*# ) + click "Sensors". It does show proximity sensor at ADC 16 or more (I think it triggers at 15, so always on). When I bring the hand to the sensor, ADC goes to 100, so the sensor is somewhat working, but it always does sense something which isn't there, which is causing it to misfire. I have another good S3, and ADC value on it is 0 or 1 when nothing is in front of it, so my sensor could clearly be better.
I have no idea what ADC is, by the way
I have another broken front panel laying around, so I took the phone apart and replaced a sensor with the one from that panel. I did see a slight improvement, which is: ADB value went down to 11-12, so the sensor was not ALWAYS in triggered state, but clearly still no bueno.
What I'm going to do now is get a new item and try replacing with a new one. It's not overly expensive, like 10 bucks when shipped from china (search for "Front Facing Camera s3" on ebay). Should that be a success, I will post my results here.
I also am not sure if it is possible to recalibrate sensor to not trigger if ADC value is, say, below 20 and such. I would imagine there could be a configurable value. Somewhere this value of 15 is coming from, isn't it? So, idk, if some guru here is reading this post, maybe could advise where to go to configure it