2003 GMC Sierra 1500 Pickup Owner's Manual - Page 161

Hazard Warning Flashers Your hazard warning flashers work no matter what position your key is in, and even if the key isn't in. Press the button to make the front and rear turn signal lamps flash on and off. Press the button again to turn the flashers off. When the hazard warning flashers are on, your turn signals won't work. Your hazard warning flashers let you warn others. They also let police know you have a problem. Your front and rear turn signal lamps will flash on and off. Other Warning Devices If you carry reflective triangles, you can use them to warn others. Set one up at the side of the road about 300 feet (I00 m) behind your vehicle. Horn 3-6