GM Service Manual Online
For 1990-2009 cars only

Leaving Your Vehicle with the Engine Running Automatic Transaxle Only

If you have to leave your vehicle with the engine running, be sure your vehicle is in Park (P) and your parking brake is firmly set before you leave it. After you've moved the shifter lever into Park (P) position, hold the regular brake pedal down. Then, see if you can move the shifter lever away from Park (P) without pressing the button on the shifter lever. If you can, it means that the shifter lever wasn't fully locked into Park (P).