“Warning: Camera Failed” shows on the Galaxy S4 after replacing the camera. The reason is that you use a wrong S4 camera. watch the video : [http://www.youtube.com/watch?v=chzX5Q2AihU&list=UUAKQbVo3Iwcp7kCqUNsd7MA|Why camera app failed after Galaxy S4 I9500 camera repairing]
The words typed on the camera flex cable is the key to distinguish the different cameras.
The camera typed “GT-I9505” or “F” on the flex cable is suitable for SCH-I545, SCH-R970, SGH-M919, SPH-L720 and SGH-I337 version. The camera flex cable was typed "GT-I9500", which is only suitable for I9500.