Chevrolet 1998 C/K Full-Size Pickup Owner's Manual page 309

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The air cleaner assembly
has
an
indicator that lets
you know when the air filter
is dirty and needs to be
serviced. The indicator is
located in the air intake tube
between the air cleaner and
the engine.
See "Owner Checks and Services" in the Index to
determine when to check the indicator.
If the area inside the clear section of the indicator is
green, no air filter service is required. When the area
inside the indicator is orange and CHANGE AIR
FILTER appears, the filter should be replaced.
To remove the air filter, unhook the retaining clips and
remove the cover. Lift the filter and the connected duct
out of the air cleaner housing. Hold the duct and remove
the filter by both pulling and twisting the filter away
from the duct. Care should be taken to dislodge as little
dirt as possible.
Clean the filter sealing surface of the duct and the
filter housing.
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