I've had this problem and in very cold temps it can freeze over. That causes the exhaust to back up and emit a massive plume of white smoke. It happened twice this winter. Volvo blames it on my driving habits. I understand that to a point, but I've had other cars and never had this problem. My wife drives her car less than I drive the XC40 and she's never had this problem. None of my friends have had this problem. None of my friends have ever heard of this problem. But I'm more conscious of it now and use remote start to let the car warm up for 10-15 minutes in very cold weather before embarking on my 6-minute commute to work. I otherwise love the car.