Cadillac CTS 2008 Owner's Manual page 196

To adjust for compass variance, use the following
procedure:
Compass Variance (Zone) Procedure
1. Do not set the compass zone when the vehicle is
moving. On an automatic transmission vehicle,
only set it when the vehicle is in PARK (P). On a
manual transmission vehicle, only set it when
the vehicle is stopped.
Press the vehicle information button until
V
Press To Change Compass Zone displays.
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2. Find the vehicle's current location and variance
zone number on the map.
Zones 1 through 15 are available.
3. Press the set/reset button to scroll through and
select the appropriate variance zone.
4. Press the trip/fuel button until the vehicle heading,
for example, N for North, is displayed in the DIC.
5. Calibrate the compass. See "Compass Calibration
Procedure" following.