Q: How to Replace the Heater Core and Perform Related Procedures in the Air Conditioning System?A: Have the air conditioning system discharged by a dealer service department or an automotive air conditioning shop before proceeding. Disconnect the cable from the negative battery terminal and drain the cooling system. Working in the engine compartment, remove the cowl cover and disconnect the heater hoses and air conditioning refrigerant lines at the firewall. Remove the instrument panel, steering column, blower motor housing mounting fasteners, and the mounting nuts for the Powertrain Control Module (PCM) bracket. Detach the ducts from the top and bottom of the heater/air conditioning housing, detach the evaporator drain hose, and remove the mounting fasteners for the housing. Disconnect the electrical connectors from the instrument panel support beam wiring harnesses and remove the instrument panel support beam mounting fasteners. Carefully pull the heater/air conditioning housing directly away from the firewall and remove it. Remove the rubber seals around both heater core tubes, the cover over the heater core tubes and the vent ducting over the heater core itself, and the small bracket fitting for one of the heater core tubes. Before installing the heater core, make sure that any foam seals on the core or housing are in place. Carefully slide the heater core into the housing, reinstall the remaining components in the reverse order of removal, refill the cooling system, and reconnect the battery. Start the engine and check for any leaks and for proper heater operation.