GMC Sonoma 2001 Owner's Manual page 381

Long Trip/Highway Scheduled Maintenance
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.
150,000 Miles (240 000 km)
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Drain, flush and refill cooling system (or every 60 months since last service,
whichever occurs first). See "Engine Coolant" in the Index for what to use.
Inspect hoses. Clean radiator, condenser, pressure cap and neck. Pressure test
the cooling system and pressure cap.
An Emission Control Service.
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