2003 GMC Sierra 1500 Pickup Owner's Manual - Page 143

In order to calibrate, CAL must be displayed in the mirror compass windows. If CAL is not displayed, push in the COMP button for approximately eight seconds or until CAL is displayed. The compass can be calibrated in one of two ways: Drive the vehicle in circles at five mph (8 km/h) or less until the display reads a direction, or 0 drive the vehicle on your everyday routine and after several turns the compass will become calibrated and will display a direction. To adjust for compass variance: 1. Find your current location and variance zone number on the following zone map. 2. Press and hold the COMP button for five seconds until a zone number appears in the display. 3. Press the COMP button on the bottom of the mirror until the new zone number appears in the display. After you stop pressing the button, the display will show a compass direction within a few seconds. Compass Variance The mirror is set in zone eight upon leaving the factory. It will be necessary to adjust the compass to compensate for compass variance if you live outside zone eight. Under certain circumstances, as during a long distance cross-country trip, it will be necessary to adjust for compass variance. Compass variance is the difference between earth's magnetic north and true geographic north. If not adjusted to account for compass variance, your compass could give false readings. 2-56