I went to have my a/c system recharged because the last mechanics discharged the system when they did head gasket work on the car. But when I took the car into another shop, they did an inspection and told me that the a/c control head was broken and needed replacement. But, if the head unit is just the controls for the a/c they were working fine when I got back into the car, but of course, not blowing cold air because they didn't recharge the system. How can I tell if the head control unit is really damaged and needing repairs?