Replacing the heater core in a Pontiac Sunfire is a very difficult and complex process. The heater core is kept within the car's heating/air conditioning module behind the instrument panel. So many other parts have to be drained and removed from the car to reach the heating/air conditioning unit that you may be better off leaving this to a professional mechanic--especially one licensed to work on air conditioning units.
Make sure the car is cool, the front wheels are facing forward and the ignition switch is off. Drain the engine coolant by removing the drain plug at the radiator and letting the coolant pour into a clean container, then do the same at the engine block's drain plug. Recover the refrigerant by connecting an air conditioning reclaimer to the system high and low side fittings as its instructions state.
Disconnect the evaporator lines to the evaporator by removing its bolt and disconnect the heater hoses from the core by loosening the hose clamps with clamp pliers. Remove the drain tube from the evaporator case. Disconnect the negative battery cable.
Remove all the trim covers and panels from the instrument panel; some of these are screwed on and some require a flat bladed tool. Remove all air distribution ducts from the panel, the air bags, the steering wheel, the radio, the tilt and washer levers and all electrical connectors, then unscrew and remove the instrument panel from the car. Unbolt the cross vehicle beam, remove its wiring harness and remove the beam.
Remove the air outlet in the floor. Disconnect the wiring harness for the heater/air conditioner and the electrical connectors at the blower's motor and resistor.
Remove the heater/air conditioning module by removing its attaching bolts and the assembly screw. Remove the heater cover case by removing its heat stakes and screws and remove the heater core.
Install the replacement core into the heater/air conditioner module, then re-connect the heater core cover case. Install the assembly in the vehicle, aligning the mounting bracket to the front of the dash slot and mounting bolt hole. Reconnect the electrical connectors and wiring harness and install the floor air outlet.
Reinstall the cross vehicle beam and every component of the instrument panel. Reconnect the negative battery cable.
Install the drain tube back on the evaporator case. Connect the hater hoses and the evaporator lines.
Recharge the air conditioner. Connect a can of R134a refrigerant to the air conditioner's low side service fitting with a recharge service hose and a high pressure gauge to the high side fitting. Open the service hose valve while the car runs with the A/C at the max and let the vapor flow until the gauge reads between 225 and 250 PSI.
Refill the cooling system at the radiator filler neck. Use fresh coolant if the old liquid is dirty in any way.
