Demand call on a miss matched carrier system with the unit on the inside being a 1998 model and the unit on the outside is a 2011. Found the unit I stop the defrost control board is not operating and needs to be replaced. The unit is also running low on Freon which is r22. I have recommended that the system be replaced due to age condition and type of refrigerant. Customer has asked to speak to a comfort adviser about possible replacement. Appointment is set for 2 PM today.