GMC 2009 Sierra - Manual (Page 181)

Notice: If you keep driving your vehicle with this
light on, after a while, the emission controls
might not work as well, your 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 your warranty.
Notice: Modifications made to the engine,
transmission, exhaust, intake, or fuel system of
your vehicle or the replacement of the original tires
with other than those of the same Tire Performance
Criteria (TPC) can affect your vehicle’s emission
controls and can cause this light to come on.
Modifications to these systems could lead to costly
repairs not covered by your warranty. This could
also result in a failure to pass a required Emission
Inspection/Maintenance test. See Accessories
and Modifications on page 5-3.
This light comes on, as a check to show it is working,
when the ignition is turned ON/RUN but the engine is not
running. If the light does not come on, have it repaired.
This light also 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
your vehicle. Diagnosis and service might be
required.
• Light On Steady — An emission control system
malfunction has been detected on your vehicle.
Diagnosis and service might be required.
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