1999 GMC Yukon Owner's Manual - Page 146

yellowblue Engine Coolant Temperature Gage This gage shows the engine coolant temperature. Malfunction Indicator Lamp (Service Engine Soon Light) (Gasoline Engine) Your vehicle is equipped with a computer which monitors operation of the fuel, ignition and emission control systems. It also provides an indicator of how hard your vehicle is working. During a majority of the operation, the gage will read 210_F (100_C) or less. If you are pulling a load or going up hills, it is normal for the temperature to fluctuate and approach the 260_F (160_C) mark. If the gage reaches the 260_F (160_C) mark, it indicates that the cooling system is working beyond its capacity. The "Problems on the Road," section of this manual shows what to do. See "Engine Overheating" in the Index. This system is called OBD II (On-Board Diagnostics-Second Generation) and is intended to assure that emissions are at acceptable levels for the life of the vehicle, helping to produce a cleaner environment. The SERVICE ENGINE SOON light comes on to indicate that there is a problem and service is required. Malfunctions often will be indicated by the system before any problem is apparent. This may prevent more serious damage to your vehicle. This system is also designed to assist your service technician in correctly diagnosing any malfunction. 2-76