The issue of the proximity sensor not working properly with a new 5S or 5C screen is the single biggest headache for me at my shop right now. This issue first cropped up for me in August. Prior to that I had no issues with the proximity sensor not working with replacement screens on the 5S and 5C. And yes, I'm positive that there were not issues prior to that because it is one of the functions I check before and after every single repair. I was using Apple N' Berry as a supplier for 5S and 5C screens so I switched to Wholesale Gadget Parts. Initially, this was effective and the problem went away, but it has since resurfaced. Here's what my countless tests have concluded:
I only see the problem on black 5S and 5C screens, not on white 5S screens.
Sometimes a pre-installed frame or hold (as it's called on the eTech Parts site) is the problem because it's position is off or because it's slightly too small. If it's too small and the proximity sensor isn't sitting extremely close or making contact with the screen, that can cause it not to work. Sometimes you can remove the frame other times you can't because there's too much glue under it. You also have to be careful when you remove it because it's easy to scratch the black paint off.
The proximity sensor will work with out it's frame or "hold" in an original Apple screen and in some replacement screens. Regardless, I always use one UNLESS there was one on the replacement screen that I remove and there too much glue from the frame I removed. So, I have go without the frame in an effort to make sure that the proximity sensor sits touching the screen. In cases like this, as long as the filter in front of the proximity sensor is causing the problem, not having the frame does not cause the proximity sensor to not work.
The rubber frame for the light sensor being in place properly, does not solve the proximity sensor issue.
The little white square light filter that sits under the light sensor should probably be moved from the original screen to the replacement screen. I always move it so I can discount it as a possible issue with affecting the proximity sensor. Does it really matter? I think it probably doesn't from my tests.
I've tried coloring the purple filter for the proximity sensor with a sharpie. I've tried scratching a tiny pin hole hole off. I've tried scratching all the purple off. I've tried coloring with sharpie after scratching a pin hole hole or scratching all the purple off. None of these worked for me.
Very often when you test, if you leave the temporary screen protector that came with the new part in place, the proximity sensor will work fine. If you cover it with your finger without the screen protector in place, it will work fine but when you put the phone up to your ear with out the screen protector in place it does not work or it does not work 100% of the time. This makes me think too much light is getting in to the sensor and causing it to not work properly. This then leads me to try coloring in the purple filter on the screen which as I said above, has yet to work for me.
The only thing that works for me is to pre-test every single screen with an original proximity sensor part and send back any screens that don't work before I get to trying to use it for a repair. This is horribly time consuming and I just can't always do it.
Both of my these suppliers have basically told me I'm the only person in the world who has this problem. I've complained more than once to each of them and they have told me that no one else has reported the issue. People either aren't testing, aren't complaining, are living with it, or the suppliers are lying. This is really frustrating.
I don't think the suppliers are going to correct this issue so it's up to us to find a work around that works 100% of the time. I hope one of us finds one soon.