1999 GMC Sonoma Owner's Manual - Page 336

yellowblue Short Trip/City Scheduled Maintenance 100,000 Miles (166 000 km) j Inspect spark plug wires. An Emission Control Service. j Replace spark plugs. An Emission Control Service. j If you haven't used your vehicle under severe service conditions listed j DATE ACTUAL MILEAGE SERVICED BY: previously and, therefore, haven't changed your automatic transmission fluid, change both the fluid and filter. Manual transmission fluid doesn't require change. 4.3L V6 Engine Only: Inspect Positive Crankcase Ventilation (PCV) valve. An Emission Control Service. DATE ACTUAL MILEAGE 150,000 Miles (240 000 km) j 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 cooling system and pressure cap. An Emission Control Service. SERVICED BY: 7-28