I have the same issue with my iphone. Still haven't tried the sharpy pen method. Also checking the sensor with a camera is a good idea thou I'm afraid I might not see no light. Strange thing is this guy on you tube was trying so hard to show how he had fixed it and you could see the light from the sensor on the screen very clearly made me giggle.
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== Update ==
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Sucess my light approximity finally works. So the tape didn't work for me I though of using some foam because I wanted it like the original. Foam did work but I had to place it in the middle of the two sensors.A vertical strip note it has to touch the front of the screen or it won't work so make sure the foam is bigger then any old one there. I'm shocked it worked for me as I have a 4s screen on an iphone 4 I got of ebay.yeh I had to change the bazel front. For more info see link.
I have the same issue with my iphone. Still haven't tried the sharpy pen method. Also checking the sensor with a camera is a good idea thou I'm afraid I might not see no light. Strange thing is this guy on you tube was trying so hard to show how he had fixed it and you could see the light from the sensor on the screen very clearly made me giggle.