If your A/C system is working OK at highway speeds but seems to fizzle out when in traffic or at a stop light it might be your aux fan resistor. When this part fails there is not enough air flowing through the radiator to lower the refrigerant temps. The resistor is located on the drivers side of the radiator (you may need to remove the headlight to get to it). The part number is 4AO 959 493 and ETKA calls it a series resistor for vehicles with heavy-duty water cooling system and/or trailing towing – ETKA list price is $92.00.