Carbon monoxide: Beware of carbon monoxide poisoning. Cars produce carbon monoxide, which is a deadly gas. Make sure you get enough fresh air. What not to do: Leave the engine running in a garage. Leave windows open when following another car closely. Leave the engine running and the windows closed while the car is parked. Driving with a faulty muffler or exhaust system. Using the air conditioning or heater in a car parked with the windows closed in an enclosed space. If you do come across a carbon monoxide poisoning victim, take them to fresh air and call 911.