Is your car leaking coolant? The problem is an external problem and most likely a visible leak. First perform a visual check around the engine bay. Look at the radiator fluid reservoir and ask yourself, is there enough coolant? If the fluid level is below the minimum marking then most likely you have a leak. Here is a list of place to check.
Radiator hose: Often time, coolant leaks from worn hoses. Vehicle pushing 150,000 miles and above start to experience coolant leak due to worn radiator hoses. Check the top and bottom radiator hoses. Check the reservoir hose and heater core hoses. The leak can also occur around the thermostat housing and make sure to check that as well.
Radiator cap: A worn radiator cap and slowly leak fluid. In fact, the cap can be worn to the point that it can no longer hold pressure as well. Replacing the cap is inexpensive and quick fix.
Water pump: Coolant can leak around the water pump due to worn gasket and seal. The gasket function is to prevent leakage between pump shaft and pump housing.
Damage component: Sometime a damaged component can cause such leak. For example, an object hitting the bottom of the radiator can cause it to crack and leak.
Repair mistake: Sometime a hose is replaced but clamp is not tightened properly.