2006 Chevrolet Express Van Owner's Manual - Page 170

To adjust both tone controls and both speaker controls to the middle position, first end out of audio mode by pressing another button, causing the radio to perform that function, or by waiting five seconds for the display to return to time of day. Then press and hold AUDIO for more than two seconds until you hear a beep. CEN will appear on the display. Radio with CD Radio Messages CAL (Calibration): The audio system has been calibrated for your vehicle from the factory. If CAL appears on the display it means that the radio has not been configured properly for your vehicle and must be returned to the dealer for service. LOC (Locked): This message is displayed when the THEFTLOCK® system has locked up. Take the vehicle to the dealer for service. If any error occurs repeatedly or if an error cannot be corrected, contact your dealer. Playing the Radio PWR (Power): Press this knob to turn the system on and off. VOL (Volume): Turn this knob to increase or to decrease the volume. RCL (Recall): Press this knob to switch the display between the radio station frequency and the time. When the ignition is off, press this knob to display the time. 3-48