I was quite impressed with today's episode. The writers' did their research and procured a plausible cause of death, hypercapnia or death by carbon dioxide via dry ice, a solid substance composed of CO2. While not usually toxic, in a closed system aka a car, where no substances/molecules can exist or enter, dry ice may cause hypercapnia, a buildup of carbon dioxide in the blood.
Quite honestly, as soon as I saw the dude die in the car, I immediately thought carbon monoxide. So, I for one, was pleasantly surprised the writers did not go down that route.