1997 GMC Sonoma Owner's Manual - Page 108

Malfunction Indicator Lamp (Service Engine Soon Light) Your vehicle is equipped with a computer which monitors operation of the fuel, ignition and emission control systems. I NOTICE: If you keep driving your vehicle with this light on, after a while, your emission controls may not work as well, your fuel economy may not be as good and your engine may not run smoothly. as This could lead to costly repairs that may not be covered by your warranty. This light should come on, as a check to show you it is working, when the ignition is on and the engine is not running. If the light doesn't come on, have it repaired. This light will also come on during amalfunction in one of two ways: SERVICE ENGINE SOON This system is called OBD I1 (On-Board Diagnostics-Second Generation) and is intended to assure that emissions areat acceptable levels for the life of the vehicle, helping to produce a cleaner I environment. (In Canada, OBD T is replaced by Enhanced Diagnostics.) 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. Light Flashing -- A misfire condition has been detected. A misfire increases vehicle emissions and may damage the emission controlsystem on your vehicle. Dealer or qualified service center diagnosis and service is required. Light On Steady -- An emission control system malfunction has been detected on your vehicle. Dealer or qualified service center diagnosis and service may be required.