GMC 2001 Sonoma Owner's Manual page 366

Short Trip/City Scheduled Maintenance
99,000 Miles (165 000 km)
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Change engine oil and filter (or every 3 months, whichever occurs first).
An Emission Control Service.
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Lubricate chassis components (or every 3 months, whichever occurs first).
(See footnote #.)
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Check rear/front axle fluid level and add fluid as needed. Check constant
velocity joints and axle seals for leaking. (See footnote **.)
100,000 Miles (166 000 km)
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Inspect spark plug wires.
An Emission Control Service.
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Replace spark plugs.
An Emission Control Service.
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If you haven't used your vehicle under severe service conditions listed
previously and, therefore, haven't changed your automatic transmission
fluid, change both the fluid and filter. Manual transmission fluid doesn't
require change.
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4.3L V6 Engine Only: Inspect Positive Crankcase Ventilation (PCV) valve.
An Emission Control Service.
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