GMC 1999 Sierra 1500 Pickup Owner's Manual page 390

Scheduled Maintenance Services
GASOLINE ENGINES WITH HEAVY DUTY EMISSIONS-
I
If your driving conditions meet those specified in "Scheduled Maintenance
FOOTNOTES:
'
An Emission Control Service
'*See "Explanation
of
Scheduled Maintenance Services" in this section.
A Also a Noise Emission Control Service
I Applicable only to vehicles sold in the United States
t
To determine the emissions classification of your engine refer to "Selecting the
Proper Maintenance Chart" in this section. TO339
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