The A/C Servo is a black box mounted in the front of the engine compartment with a lot of vavuum lines connected to it, and a few heater hoses. It controls the coolant flow for the heater/defroster. A lot of times they develop bad cracks and leaks, or the internal valves and flaps sieze up. They run anywhere from $600-$900, plus about $100 for a new climate control amp...