2001 GMC Sierra 1500 Pickup Owner's Manual - Page 441

Scheduled Maintenance -- Gasoline Engines j Inspect spark plug wires. An Emission Control Service. j Replace spark plugs. An Emission Control Service. j Change automatic transmission fluid and filter if the vehicle's GVWR 100,000 Miles (166 000 km) DATE ACTUAL MILEAGE SERVICED BY: is over 8600 lbs. or if the vehicle is mainly driven under one or more of these conditions: - In heavy city traffic where the outside temperature regularly reaches 90_F (32_C) or higher. - In hilly or mountainous terrain. - When doing frequent trailer towing. - Uses such as found in taxi, police or delivery service. j 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. j Automatic Transfer Case Only: Change transfer case fluid. j Inspect Positive Crankcase Ventilation (PCV) valve. An Emission Control Service. 7-16