Mitsubishi A/C Heater Problems?

There is no such thing as AC coolant. There is engine coolant, which the heater core uses to heat the cabin. The AC uses a compressed gas (freon). It's essentially the same type of set up as you have in your refrigerator at home. There is no way to "check" it. My guess without looking at your car though would be the blend door since you have neither heat, nor AC. The blend door controls how much air comes from outside, or flows through the heater core, or AC evaporator. It's how you adjust the temperature. This could be a problem with the control itself, a leaking vacuum line (it's a vacuum operated system), a faulty actuator on the door, or something wrong with the door itself.