GMC 2001 C3 Owner's Manual page 118

Temperature and Compass Display
Press the on/off button, located to the far left, briefly to
turn the comp/temp display on or off.
If the display reads "CAL," you will need to calibrate
the compass. For more information on calibration,
see below.
To adjust between Fahrenheit and Celsius do
the following:
1. Press and hold the on/off button for four seconds
until either a flashing _ F, or _ C appears.
2. Press the button again to change the display to the
desired unit of measurement. After approximately
four seconds of inactivity, the new unit will be
locked in and the compass/temperature display
will return.
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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 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
six seconds until the green indicator light turns off.