Upon arrival the technician found that the Plenum coil is leaking refrigerant. The system is approximately 15 years old. Technicians best recommendation would be to replace the entire system considering the age, if one part is failing now they are going to be sinking money into the system to replace more parts over the next few years until the system finally fails. The minimum option is to replace the coil in order to keep the refrigerant from escaping the system.