After I dropped my N9 I had to have the case and the display replaced. The place I took it to for the repairs put in a very cheap display.
The display started turning off so often that I could not make or receive calls. Finally after the display started separating from the bezel, then turned all green. I bought another display from a company in California through Ebay. I think I paid somewhere around $80 for the display back in 2014.
So far the phone has been working perfectly ever since.
Which makes me wonder is it the sensor or a cheap display causing problems?