1999 Chevrolet Silverado 1500 Pickup Owner's Manual - Page 158

yellowblue CHANGE ENGINE OIL This message is displayed when the engine oil needs to be changed. The message is only displayed for 15 seconds at the start of each ignition cycle. When you change the engine oil, be sure to reset the Engine Oil Life Monitor. See "Engine Oil, When to Change" in the Index. SERVICE 4WD (If Equipped) If the SERVICE 4WD message comes on, there may be a problem with the Automatic Transfer Case and service is required. SECURITY This message is displayed when the content theft-deterrent system has been activated and also monitors the PasslockR System. If the security message is displayed 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. 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. 2-86