2003 GMC Sonoma Owner's Manual - Page 302

4. Then fill the coolant recovery tank to ADD mark. 5. Put the cap back on the coolant recovery tank, but leave the radiator pressure cap off. 6. Start the engine and let it run until you can feel the upper radiator hose getting hot. Watch out for the engine cooling fan. 7. By this time, the coolant level inside the radiator filler neck may be lower. If the level is lower, add coolant mixture more of the proper DEX-COOL@ through the filler neck until the level reaches the base of the filler neck. 5-39