2004 Cadillac CTS Owner's Manual - Page 164

Engine Coolant Temperature Gage This gage shows the engine coolant temperature. If the gage pointer moves into the shaded area, the engine is too hot. Malfunction Indicator Lamp Check Engine Light Your vehicle is equipped with a computer which monitors operation of the fuel, ignition and emission control systems. That reading means the same thing as the warning light - the engine coolant is very hot. See Engine Overheating on page 5-30. This system is called OBD II (On-Board DiagnosticsSecond 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 Check Engine 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. 3-42