2012 GMC Sierra Owner's Manual - Page 302

9-26 Driving and Operating will remain active. See Retained Accessory Power (RAP) on page 9‑29. This position locks the ignition. It also locks the transmission on automatic transmission vehicles. The key can be removed in LOCK/OFF. Do not turn the engine off when the vehicle is moving. This will cause a loss of power assist in the brake and steering systems and disable the airbags. 3. Come to a complete stop, shift to P (Park), and turn the ignition to LOCK/OFF. On vehicles with an automatic transmission, the shift lever must be in P (Park) to turn the ignition switch to the LOCK/OFF position. 4. Set the parking brake. See Parking Brake on page 9‑54. Ignition Positions { WARNING Turning off the vehicle while moving may cause loss of power assist in the brake and steering systems and disable the airbags. While driving, only shut the vehicle off in an emergency. If the vehicle cannot be pulled over, and must be shut off while driving, turn the ignition to ACC/ ACCESSORY. The ignition switch has four different positions. To shift out of P (Park), the ignition must be in ON/RUN or ACC/ ACCESSORY and the regular brake pedal must be applied. A (STOPPING THE ENGINE/ LOCK/OFF): When the vehicle is stopped, turn the ignition switch to LOCK/OFF to turn the engine off. Retained Accessory Power (RAP) If the vehicle must be shut off in an emergency: 1. Brake using a firm and steady pressure. Do not pump the brakes repeatedly. This may deplete power assist, requiring increased brake pedal force. 2. Shift the vehicle to N (Neutral). This can be done while the vehicle is moving. After shifting to N (Neutral), firmly apply the brakes and steer the vehicle to a safe location.