GMC YUKON XL 2003 Manual page 132

Electrochromic Mirror Operation
The electrochromic (self dimming) mirror function is
turned on automatically each time the ignition is started.
To operate the electrochromic mirror do the following:
1. Make sure the green indicator light, located to
the left of the on/off button, is lit. If it's not, press
and hold the on/off button for approximately
six seconds until the green light comes on,
indicating that the mirror is in electrochromic (self
dimming) mode.
2. Turn off the electrochromic mirror function by
pressing and holding the on/off button for
approximately six seconds until the green indicator
light turns off.
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 of 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 do the following:
1. Find your current location and variance zone
number on the following zone map.
2. Press and hold the on/off button until a Z and a
zone number appears in the display. The compass
is now in zone mode.
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