My car is OPC, hence I seldom drive during the day with the Yi camera. Whenever I park at night, I disconnect the cam and keep it in the glove compartment so that the sun does not directly heat up the camera.
My online research shows that the camera focus tends to blurry under strong heat, so while I'm working during the day I make sure my Yi camera is not connected in the car while it's parked. Just a preventive measure. However, if this happens while driving, the focus will be back to normal after the camera cools down.
Using the Yi cam for a few months now, absolutely happy for the price. Can't complain at all.