A Chevrolet heater core is the unit in your car's interior where the heat will come from in the cold winter months. The heat that emanates from this core is provided by the lines which are connected to the radiator on your Chevrolet. There are two reasons the heater core on your Chevrolet would need to be replaced. The first would be if there was a coolant leak found in the radiator in the past that was "fixed" by adding a leak-stop additive. This, however, can cause its own problems in the cooling system because not all of the components are designed to accept this chemical. The Chevrolet heater core has very small channels and the components of the leak-stop additive will block these channels preventing the flow of hot coolant through it, thus no heat for the passenger compartment. The second reason for the replacement of your heater core would be because it is leaking. This leak is generally noticed when coolant is found on the floor board on the front passenger side of the vehicle. Because the channels in this part are so small that the coolant travels through, a stop leak additive will stop the leak in most cases but also prevent the heating element from having any coolant travel through it. Replacement of the part will be required if you desire heat in the passenger compartment. The replacement of the heater core on most Chevrolet models requires the partial disassembly of the dashboard unit in the interior of the vehicle. This is a time consuming process but necessary since this component is a vital part of the temperature control system for your car. It is not only used when heat is desired but also required for a properly working defroster.