2001 GMC Sierra 1500 Pickup Owner's Manual - Page 447

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. Park on a fairly steep hill, with the vehicle facing downhill. Keeping your foot on the regular brake, set the parking brake. D With an automatic transmission, the key should turn to LOCK only when the shift lever is in PARK (P). D To check the parking brake's holding ability: D With a manual transmission, the key should turn to LOCK only when you press the key release button. 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. On all vehicles, the key should come out only in LOCK. Parking Brake and Automatic Transmission PARK (P) Mechanism Check D To check the PARK (P) mechanism's holding Underbody Flushing Service 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. 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. 7-22