camera failed :(

i bought a att note 3 couple of months ago box cams are blurry so i noticed some dust inside . i open the phone clear the dust so front cam is works like a charm but back camera isn't working with an error of camera failed . clearing app data isn't help , re-installing new firmwares(kitkat,lollipop both) also came with same error, so i visit for some repair shops and try more than a dozen of camera pieces but camera failed isn't gone. so is there anyone who know whats wrong with it and how i get out from this?
thanks in advance for your time and help.