GMC Sierra Denali Owner's Manual page 388

Ignition Transmission Lock Check
While parked, and with the parking brake set, try to turn
the ignition key to LOCK in each shift lever position.
*
The key should turn to LOCK only when the
The key should come out only in LOCK.
shift lever is in PARK (P).
Parking Brake and Automatic
Transmission Park (P) Mechanism
A .
le1
,
..
When you are doing this check, your vehicle
could begin to move. You or others could be
injured and property could be damaged. Make
sure there is room in front of your vehicle in
case it begins to roll. Be ready to apply the
regular brake at once should the vehicle begin
to move.
Park on a fairly steep hill, with the vehicle facing
downhill. Keeping your foot on the regular brake, set the
parking brake.
@
To check the parking brake's holding ability: With
the engine running and transmission in NEUTRAL
(N), slowly remove foot pressure from the regular
brake pedal. Do this until the vehicle is held by the
parking brake only.
ability: With the engine running, shift to PARK (P).
Then release the parking brake followed by the
regular brake.
To check the PARK (P) mechanism's holding
Underbody Flushing Service
At least every spring, use plain water to flush any
corrosive materials from the underbody. Take care to
clean thoroughly any areas where mud and other debris
can collect.
6-1 3