GMC 2003 GMT560 T-Series Owner's Manual page 170

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.
5-14
If equipped, the restriction
indicator will be located in
the engine compartment
near the engine air
cleaner/filter, on the
passenger's side of the
vehicle.
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CAUTION:
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 helps to
stop 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: 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.
For more information regarding checking your restriction
indicator, see Part B: Owner Checks and Services on
page 6-22 .
Your vehicle may be equipped with an optional
instrument panel mounted air filter restriction indicator.
See Air Filter Restriction Indicator on page 3-38 .