GMC SIERRA CK2 1999 Owner's Manual page 300

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.
6-18
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.
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.
Install the new filter by pushing it all the way to the stop
on the duct.
Install the duct and the filter into the air cleaner housing.
Make sure that the duct fits properly into the housing.
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