GMC 2005 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.
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