1999 GMC Sonoma Owner's Manual - Page 349

yellowblue Long Trip/Highway Scheduled Maintenance 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 the cooling system and pressure cap. An Emission Control Service. DATE ACTUAL MILEAGE SERVICED BY: 7-41