GMC TopKick 2007 Owner's Manual page 278

How to Inspect the Engine Air
Cleaner/Filter
Locate the filter change indicator. When the
change indicator turns black or is in the red/orange
change area, replace the filter and reset the
indicator.
To inspect the engine 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.
278
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 flames 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.