Diagnostics
OnStar Vehicle Diagnostics will perform a vehicle check every month. It will check the engine, transmission, antilock brakes, and major vehicle systems.
It also checks the tire pressures, if the vehicle is equipped with the Tire Pressure Monitoring System.
If a diagnostics check is needed between e-mails, push 
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Advisor can run a check.
See also:
Braking
Braking action involves perception time and reaction time. Deciding to push
the brake pedal is perception time. Actually doing it is reaction time.
Average driver reaction time is about three-quar ...
Rear Compartment Fuse
Block (CTS Wagon)
This fuse block is on the right side
of the vehicle, behind the trim panel
in the rear compartment.
The vehicle may not be equipped
with all of the fuses, relays, and
features shown.
...
If the System Needs Service
If the navigation system needs service and the steps listed here have been followed
but there are still problems, see your dealer for assistance. ...