GMC Sierra Classic 2007 Owner's Manual page 624

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 ignition key should come out only in
LOCK.
Contact your GM Goodwrench
required.
624
®
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.