Don't forget to check for stuck pixels and dead pixels.
Dead Pixels:
Manually focus and take a picture of a white wall/paper, try a 2-3 second exposure. Check the image to see if there are any dead (black) pixels.
Stuck Pixels:
Keep the lens cap on, manual focus, try a 5 second exposure at various ISOs. You'll probably see some hot pixels, having some at higher ISOs is pretty much the norm. Take note of the location of the hot pixels, if they're still showing up when taking a normal picture, it may be stuck.
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