About a month ago, the GPS function on my Iphone 6 started to fail. I tried all kinds of software reset options including wiping out IOS completely, nothing worked. The genius bar did not help either, and suggested phone replacement.
A quick Google search showed similar problems of other Iphone 6 customers and pointed me to potential hardware problems with antenna flex cable/wifi antenna.
1. I purchased a piece of antenna flex cable from an Amazon seller, it did not improve anything.
2. I also bought a wifi antenna, it did not do a thing
3. Before giving up, I bought both antenna flex cable and wifi antenna from Ifixit. This time it worked!
While struggling with my GPS problem, I read through some related Ifixit discussion threads and noticed someone mentioned the missing or damaged "sticker" for the flex cable. I am not sure about the exact purpose of the stickers, but ebay http://www.ebay.com/itm/162377984938 gave me some hint that the original part might have stickers on both sides of the cable. Those stickers might help position the flex cable or have better connection to the metal pieces above and below, just my guess. When I received the Ifixit flex cable, it looks professionally packaged, but it does not have any sticker. I happened to have one sticker from a broken cable, I attached it to the bottom side (facing the phone case). The phone now works perfectly and the wifi signal improves a lot as well.
I am not really sure which part really fixed the problem, but feel having at least one sticker is important. I still bought the ebay part as a backup plan, hopefully it has stickers on both side as the picture showed.