2004 ford freestar air conditioning com...

Actually, it is very common to be low on coolant. In fact that is the most common cause of your problem. If you are not comfortable doing this your self you will need to go to a shop and have the AC serviced. It is fairly easy. GO to Auto Zone and purchase one of the refill or cool boost cans with a gauge on it. Locate the low pressure side, this is easy in that the fitting on the can will only fit on the low pressure side. Start the vehicle and set the AC to MAX cool. Follow the directions on the can to fill your system. R134a works on pressure in the system. If the pressure falls the cooling falls that is why you felt the cooling better when driving on the highway than you did in traffic. Make sure you don't overfill the system. If you do it is possible that you will damage a seal and end up having to replace the compressor. Good Luck.