2002 GMC Sonoma Owner's Manual - Page 147

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 a malfunction in one of two ways: If the Light Is Flashing The following may prevent more serious damage to your vehicle: D Light Flashing -- A misfire condition has been detected. A misfire increases vehicle emissions and may damage the emission control system on your vehicle. Dealer or qualified service center diagnosis and service may be required. D D D D Reducing vehicle speed. Avoiding hard accelerations. Avoiding steep uphill grades. If you are towing a trailer, reduce the amount of cargo being hauled as soon as it is possible. D 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. If the light stops flashing and remains on steady, see "If the Light Is On Steady" following. If the light continues to flash, when it is safe to do so, stop the vehicle. Find a safe place to park your vehicle. Turn the key off, wait at least 10 seconds and restart the engine. If the light remains on steady, see "If the Light Is On Steady" following. If the light is still flashing, follow the previous steps, and drive the vehicle to your dealer or qualified service center for service. 2-77