1999 GMC Sonoma Owner's Manual - Page 123

yellowblue 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 dealer. Also see "Content Theft-Deterrent" in the Index for additional information regarding the SECURITY light. The SERVICE 4WD light comes on to indicate that there may be a problem with the 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. Check Gages Light If this light comes on and stays on while you are driving, check your coolant temperature and engine oil pressure gages to see if they are in the warning zones. Service Four-Wheel-Drive Warning Light This light should come on briefly when you turn on the ignition, as a check to show you it is working. 2-65