GMC Yukon Denalid Owner's Manual page 560

Ignition Transmission Lock Check
While parked, and with the parking brake set, try
to turn the ignition to LOCK in each shift lever
position.
•
The ignition should turn to LOCK only when
the shift lever is in PARK (P).
•
The key should come out only in LOCK.
Contact your GM Goodwrench
required.
560
®
dealer if service is
Parking Brake and Automatic Transmission
Park (P) Mechanism Check
CAUTION:
{
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.