GMC 2007 Sierra Owner's Manual page 170

Temperature and Compass Display
Press the on/off button, located to the far
left, briefly to turn the compass/temperature
display on or off.
If the display reads CAL, the compass needs to
be calibrated. For more information, see "Compass
Calibration" following.
170
To adjust between Fahrenheit and Celsius, do the
following:
1. Press and hold the on/off button for
approximately 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.
If an abnormal temperature reading is displayed
for an extended period of time, see your
dealer. Under certain circumstances, a delay in
updating the temperature is normal.