GMC 2009 Sierra 1500 Crew Cab Owner's Manual page 230

If the check engine light comes on and stays on, while
the engine is running, this indicates that there is an
OBD II problem and service is required.
Malfunctions often are indicated by the system before any
problem is apparent. Being aware of the light can prevent
more serious damage to the vehicle. This system assists
the service technician in correctly diagnosing any
malfunction.
Notice: If the vehicle is continually driven with this
light on, after a while, the emission controls might
not work as well, the vehicle's fuel economy might
not be as good, and the engine might not run as
smoothly. This could lead to costly repairs that
might not be covered by the vehicle warranty.
Notice: Modifications made to the engine,
transmission, exhaust, intake, or fuel system of the
vehicle or the replacement of the original tires with
other than those of the same Tire Performance
Criteria (TPC) can affect the vehicle's emission
controls and can cause this light to come on.
Modifications to these systems could lead to
costly repairs not covered by the vehicle warranty.
This could also result in a failure to pass a
required Emission Inspection/Maintenance test.
See Accessories and Modifications on page 5-4.
3-46
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.
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
key off, wait at least 10 seconds, and restart the engine.
If the light is still flashing, follow the previous steps and
see your dealer/retailer for service as soon as possible.