Sleeping in a car with the heater on is very dangerous. Leaving the engine running overnight poses a risk of carbon monoxide poisoning, whether the car is parked in an open area or in a relatively enclosed space (such as a garage). There is always a chance of carbon monoxide poisoning. Probability of poisoning when sleeping in a car with the heater on: A car is not an airtight space. Even with all windows closed, it cannot be completely sealed. For example, areas like the air vents, under the rear fender covers, around the steering column, and near the brake and accelerator pedals are not sealed. Therefore, even with the engine off and all windows closed, the car is not airtight, and outside air can still enter. Although the ventilation efficiency decreases, it is sufficient for several adults (you won’t suffocate—just breathe near the air vent if needed). However, if the engine is left running, these openings become pathways for carbon monoxide to enter the car. Thus, sleeping in a car overnight with the windows closed and the engine running significantly increases the risk of fatal carbon monoxide poisoning. I strongly advise everyone never to attempt this. When the car is idling: When the car is idling, the engine’s combustion is less efficient, leading to a higher proportion of incomplete fuel combustion. This produces much more carbon monoxide than when the car is moving. If this carbon monoxide enters the car and is inhaled, the person may become too weak to even open the door.
