How To Fix Proximity Sensors For IPhone 4
Since the iOS 4.1 release, there have been many questions as to whether or not the update resolved the iPhone 4 proximity sensor issue once and for all. Some of you may be wondering, "What is a proximity sensor and what function does it serve?" Without getting too involved, every iPhone (including the original iPhone 2G, 3G and 3GS models) have a proximity sensor. This sensor emits an infrared (IR) light that detects the presence of a solid object (such as a head or hand), while you have a call in progress, or while you are listening to a voicemail. When it detects a solid object, it turns your iPhone's LCD panel off. This is a function you'll probably never miss until it's no longer working.
The function of the proximity sensor actually has two distinct advantages. First, you aren't muting your phone, accidentally pushing buttons, or hanging up on your callers with your cheek. The other advantage is that it conserves power consumption and improves your phone's battery life in between charging cycles.
As an Apple Certified Macintosh Technician who makes a living supporting and fixing Apple products, this is an issue that could not be ignored. The reputation of my business hung in the balance of being able to find a reliable and professional solution for this problem.
In the four years that I've been repairing iPhones, it's only been recently that I've began experiencing the problems that have been widely documented and reported. Since the release of iOS 4.1, I haven't seen any cases in which the iPhone's proximity sensor malfunctioned on a phone that hadn't been modified. When I say "modified" I mean a phone that has undergone a color conversion, or one that the front glass and LCD had been replaced due to the phone being dropped and having the original glass broken. I noticed that the occurrence was more apparent after a color conversions had been performed.
Since the proximity sensor on the iPhone 4 is located on the iPhone's mid-frame and is not a part of the front glass and LCD assembly, I had never questioned the quality of the parts I was purchasing. With that in mind, I began testing every phone prior to repair. I wanted to be sure that the phones were functioning properly, prior to repairs, or color conversions being made. So far, I have yet to find one that wasn't working just fine before the repair. It turns out, the problem was attributed to two factors.
1. The unique sensitivity of each iPhones proximity sensor.
2. The proximity sensor cover on some of the replacement glass is not tinted as dark. This allows too much light to pass through.
In fact, the best solution for this problem is an inexpensive "sensor cover." This cover is a small piece of film with a coating that looks to be the same red polarized coating on some sunglasses. This small piece of film is installed over the top of the iPhone's proximity sensor. At the end of the day, it would appear that too much light is getting in and confusing the phone. The proximity sensor cover filters excessive light from entering the phone and there by  solves the problem.