This is what worked for me. I have no idea what steps are essential. The camera on my zenfone was dead. Every diagnostic said it was dead. I did a factory reboot. The camera was still dead. A couple weeks later, I rebooted the phone. I laid the phone flat on a surface so that the lens would only see dark. I fired up the camera app. Let it sit for a minute. Went from the rear-facing to the front-facing setting a couple times. I picked the camera up, and for a few days it has been working fine.