This light comes on during a malfunction in one of two ways: Light Flashing: A misfire condition has been detected. A misfire increases vehicle emissions and could damage the emission control system on the vehicle. Diagnosis and service might be required.
GMC Acadia/Acadia Denali Owner Manual - 2011 Black plate (19,1) Instruments and Controls 5-19
To prevent more serious damage to the vehicle: . Reduce vehicle speed. . Avoid hard accelerations. . Avoid steep uphill grades. . If towing a trailer, reduce the amount of cargo being hauled as soon as it is possible. If the light continues to flash, when it is safe to do so, stop the vehicle. Find a safe place to park the vehicle. Turn the vehicle off, wait at least 10 seconds, and restart the engine. If the light is still flashing, follow the previous steps and see your dealer for service as soon as possible.