2004 GMC Sierra 1500 Pickup Owner's Manual - Page 153

The compass can be calibrated by driving the vehicle in circles at five mph (8 km/h) or less until the display reads a direction. 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. 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. Passenger Air Bag Indicator Your mirror may be equipped with a passenger air bag indicator on the mirror glass, just above the buttons. If you have this feature the mirror will display the word ON, or an airbag symbol in Canada, when the passenger air bag is enabled. For more information see Passenger Sensing System on page 1-81 2-59