Q: How to Replace a Heater Core?A: To disassemble the air conditioning system, begin by having an air conditioning technician discharge and recover the refrigerant. After disconnecting the negative battery cable, drain the cooling system and disconnect the air conditioning evaporator lines at the firewall. Mark and detach the heater control valve cable, turning the valve arm as required. Disconnect the heater hoses from the heater core at the firewall, and remove the heater/air conditioning unit mounting nut from the engine compartment firewall. Proceed to remove the instrument panel, disconnect electrical connectors, and detach the heater valve control cable if present. On 2004 and later models, lift the tab and remove the cable end from the bracket. On 2003 and earlier models, locate the heater control cable beneath the heater/air conditioning unit. For 2004 and later models, remove the unit by undoing the mounting fasteners, then lift it out. For 2003 and earlier models, remove the evaporator housing, followed by the heater core housing. Carefully lift out the heater core after dividing the housing. During reinstallation, ensure correct placement of the evaporator drain tube and refrigerant line fittings with new O-ring seals. Reconnect all electrical connectors, including the heater control valve cable using the index marks. Reattach the heater hoses, and follow the reverse order of removal, including reconnecting the battery, refilling the cooling system, and having the air conditioning system evacuated, recharged, and leak-tested by professionals.