2005 GMC Envoy Owner's Manual - Page 119

Such interference may be caused by a magnetic antenna mount, magnetic note pad holder, or a similar magnetic item. If the letter C should ever appear in the compass window, the compass may need calibration. Press and hold the on/off button for nine seconds to activate the compass calibration mode. CAL will be displayed in the compass window on the mirror. The mirror can be calibrated by driving the vehicle in circles at 5 mph (8 km/h) or less until the display shows a direction. Compass Variance Compass variance is the difference between earth's magnetic north and true geographic north. The mirror is set to 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 conditions, such as during a long-distance, cross-country trip, it will be necessary to adjust for compass variance. If not adjusted to account for compass variance, the compass could give false readings. To adjust for compass variance do the following: 1. Find the current location and variance zone number on the following zone map. 2. Press and hold the on/off button until a zone number appears in the display. 3. Once a zone number appears in the display, press the on/off button quickly until the correct zone number appears in the display. If C appears in the compass window, the compass may need calibration. See "Compass Calibration" explained previously. 2-45