2003 Chevrolet Tahoe Owner's Manual - Page 476

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 shift lever is in PARK (P). The key should come out only in LOCK. 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. To check the PARK (P) mechanism's holding ability: With the engine running, shift to PARK (P). Then release the parking brake followed by the regular brake. Parking Brake and Automatic Transmission Park (P) Mechanism Underbody Flushing Service 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. 6-13