1998 GMC Sonoma Owner's Manual - Page 226

When you decide it's safe tolift the hood, here's what you'll see: The coolant level should be at least up to the ADD mark. A. Coolant Recovery Tank If it isn't, you may have a leak inthe radiator hoses, heater hoses, radiator, water pump or somewhere else in the cooling system. B. Radiator Pressure Cap C. Engine Fan If the coolant inside the coolant recovery tank is boiling, don't do anything else until it coolsdown. 5-14