Abstract: A dual use of a heater core that enables heating the cabin, cooling the engine or both on demand regardless of the passenger's cabin heating and cooling requirements. This use of the heater core is enabled by an HVAC airbox system with a cooling door that can be selectively positioned such that at least some of the air moving through the heater core is directed to the underhood area of a vehicle thereby providing supplemental engine cooling on demand regardless of the passenger's cabin heating and cooling requirements. The cooling door can be positioned automatically by the Engine Control Unit (“ECU”) dependent on any parameter, or combination of parameters, of the engine such as the engine coolant temperature or the engine oil temperature. The blower speed and the position of the cooling door are adjusted by the ECU depending on the whether and how much supplemental engine cooling is required.