Proximity sensor not working, But other sensor works fine.

I bought an iPhone 5 CDMA off of eBay a while back, and My proximity sensor will just not work on calls or anything else. But the light sensor and front camera work just fine. I have also confirmed the proximity sensor itself works by using the screen in another iPhone 5 and placing a call. For some reason though, it won't work it mine. Idk What to do, I have iTunes restored the firmware and all that stuff, but nothing. Any ideas?