Back camera not working. (Replacement not works also)

So I bought a used iPhone 5 with back camera not working. I thought I will replace the camera and sell it for the profit. The problem is, that I replaced the camera, and it still doesn't work. I thought maybe the replacement was broken too, so I found one more camera, replaced it again and IT STILL DID NOT WORKED. Decided to use it for a couple of days. After 2 hours it started to work. By then I thought everything is going to be OK, but after a couple days it is not working again.. Been using it with defective camera for 3 days now. Flash is not working too. In the camera app it displays yellow triangle in the upper left corner, which says "The iPhone needs to cool down to use the flash" (something like that). What could the problem be???