A group of teenagers decided to use Zack's old clunker to ride to a party on a c

A group of teenagers decided to use Zack's old clunker to ride to a party on a cold winter night. The old auto had a failing engine and was also very noisy because the exhaust system was rusting out. While they were at the event, a heavy snowstorm dropped three feet of snow over the countryside with drifts quite a bit higher. As Zack rounded a curve late that night, he apparently skidded on some black ice and plowed into a high drift. Unable to get the car out of the drift, they decided to spend the rest of the night in the car with the engine running to stay warm. The next morning, they were all found dead without a mark of violence on them. What happened?