2003 Chevrolet Venture Thermostat

Was the coolant actually low? If so, suspect a leaking cylinder head gasket. There's two tests for that. Does the radiator fan turn on? If that is the cause of the overheating, it will not overheat at highway speeds where natural air flow is sufficient for proper cooling. If it only overheats above a certain speed, you might suspect a corroded radiator. Check the cooling fins to see if they crumble easily. If they do, they won't give up the heat to the surrounding air.

Feel the two radiator hoses. The top one should be hot and the bottom one should be a lot cooler. Feel across the radiator to see if there's a cold spot where coolant flow is blocked.