2003 GMC Sierra 1500 Pickup Owner's Manual - Page 226

To adjust both tone controls and both speaker controls to the middle position, end out of audio by waiting for the display to change to the time display. Then press and hold AUDIO for more than two seconds until you hear a beep. CEN will appear on the display. Fade may not be available if you have a regular cab model. Radio with CD Radio Messages CAL (Calibrated): Your audio system has been calibrated for your vehicle from the factory. If CAL appears on the display it means that your radio has not been configured properly for your vehicle and must be returned to the dealership for service. Base Radio Shown, Base@ Similar LOC (Locked): This message is displayed when the Theftlock@ system haslocked up. Your vehicle must be returned to the dealership for service. If your vehicle is equipped with the Base@ audio system, your vehicle will have six Base@ amplified speakers. Radio Data System (RDS): Your audio system is equipped with a Radio Data System (RDS). RDS features are available for use only on FM stations that broadcast RDS information. 0 Seek to stations broadcasting the selected type of programming, receive announcements concerning local and national emergencies, display messages from radio stations, and 0 seek to stations with traffic announcements. 3-7 1