iPhone 4s camera not focusing after a camera unit replacement

Hi,

My iPhone's camera focus stopped working, I did some research and found out many people solved the problem by hitting the phone at the back and front or do a full restore and start as a new phone but any of that does not solved my problem. So I decided to change the camera unit, but even this does not helped.

My question is that is there any chance that the replacement camera unit is a faulty as well (and I have to buy another one) or there are any other reasons the camera should not focusing?

Thanks for your help.