GMC 2003 C4C042 Owner's Manual page 285

When the yellow indicator reaches the red ring in the
sight glass, it's time to service the engine air
cleaner/filter.
After the engine air cleaner/filter is properly serviced,
the indicator can be reset by pressing the button on top
of the indicator.
The restriction indicator
is located in the engine
compartment near the
engine air cleaner/filter,
on the passenger's side of
the vehicle.
CAUTION:
Gasoline Engines Only: Operating the engine
with the air cleaner/filter off can cause you or
others to be burned. The air cleaner not only
cleans the air, it stops flame if the engine
backfires. If it isn't there, and the engine
backfires, you could be burned. Don't drive with
it off, and be careful working on the engine with
the air cleaner/filter off.
NOTICE:
Gasoline Engines Only: If the air cleaner/filter is
off, a backfire can cause a damaging engine fire.
And, dirt can easily get into your engine, which
will damage it. Always have the air cleaner/filter
in place when you're driving.
Your vehicle may be equipped with an optional instrument
panel mounted air filter restriction indicator. See "Warning
Lights, Gages and Indicators" in the Index.
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