2012 GMC Sierra Owner's Manual - Page 310

9-34 Driving and Operating Running the Vehicle While Parked It is better not to park with the engine running. But if you ever have to, here are some things to know. { WARNING It can be dangerous to get out of the vehicle if the automatic transmission shift lever is not fully in P (Park) with the parking brake firmly set. The vehicle can roll. Do not leave the vehicle when the engine is running unless you have to. If you have left the engine running, the vehicle can move suddenly. You or others could be injured. To be sure the vehicle will not move, even when you are on fairly level ground, always set the parking brake and move the shift lever to P (Park). { WARNING Four-wheel drive vehicles with the transfer case in N (Neutral) will allow the vehicle to roll, even if the automatic transmission shift lever is in P (Park). So, be sure the transfer case is in a drive gear - not in N (Neutral). Always set the parking brake. Follow the proper steps to be sure the vehicle will not move. See Shifting Into Park on page 9‑30. If pulling a trailer, see Driving Characteristics and Towing Tips on page 9‑74. { WARNING Idling a vehicle in an enclosed area with poor ventilation is dangerous. Engine exhaust may enter the vehicle. Engine exhaust contains carbon monoxide (CO) which cannot be seen or smelled. It can cause unconsciousness and even death. Never run the engine in an enclosed area that has no fresh air ventilation. For more information, see Engine Exhaust on page 9‑33.