GMC 2005 T-Series Owner's Manual page 171

To inspect the air cleaner/filter, remove the filter from
the vehicle and lightly shake the filter to release
loose dust and dirt. If the filter remains caked with dirt,
a new filter is required.
To inspect or replace the filter and reset the indicator
(if equipped), do the following:
1. Remove all three finger screws and carefully
remove the filter cover.
2. Inspect or replace the engine air cleaner/filter.
3. Be sure that all three finger screws are tightened
securely when reinstalling the filter cover.
CAUTION:
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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 is not
there and the engine backfires, you could be
burned. Do not 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 are driving.
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