2012 GMC Sierra Owner's Manual - Page 309

Driving and Operating When less power is required, such as cruising at a constant vehicle speed, the system will operate in the half cylinder mode, allowing the vehicle to achieve better fuel economy. When greater power demands are required, such as accelerating from a stop, passing, or merging onto a freeway, the system will maintain full-cylinder operation. If the vehicle has an Active Fuel Management™ indicator, see Driver Information Center (DIC) on page 5‑30 for more information on using this display. 9-33 Engine Exhaust WARNING (Continued) . { WARNING Engine exhaust contains carbon monoxide (CO) which cannot be seen or smelled. Exposure to CO can cause unconsciousness and even death. Exhaust may enter the vehicle if: . There are holes or openings in the vehicle body from damage or aftermarket modifications that are not completely sealed. If unusual fumes are detected or if it is suspected that exhaust is coming into the vehicle: . The vehicle idles in areas with poor ventilation (parking garages, tunnels, deep snow that may block underbody airflow or tail pipes). The exhaust smells or sounds strange or different. The exhaust system leaks due to corrosion or damage. The vehicle exhaust system has been modified, damaged or improperly repaired. (Continued) Drive it only with the windows completely down. Have the vehicle repaired immediately. . . . Never park the vehicle with the engine running in an enclosed area such as a garage or a building that has no fresh air ventilation. .