1999 GMC Suburban Owner's Manual - Page 150

yellowblue Security Light This light will come on briefly when you turn the key to START. Service Four-Wheel Drive Warning Light (If Equipped) If your vehicle is equipped with the automatic four-wheel-drive transfer case, it has a computer which controls the four-wheel-drive system. This light should come on briefly when you turn on the ignition, as a check to show you it is working. The light will stay on until the engine starts. If the light flashes, the PasslockR System has entered a tamper mode. If the vehicle fails to start, see "Passlock" in the Index. If the light comes on continuously while driving and stays on, there may be a problem with the Passlock System. Your vehicle will not be protected by Passlock, and you should see your GM dealer. The SERVICE 4WD light comes on to indicate that there may be a problem with the automatic four-wheel-drive system and service is required. Malfunctions can be indicated by the system before any problem is apparent, which may prevent serious damage to the vehicle. This system is also designed to assist your service technician in correctly diagnosing a malfunction. 2-80