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there is a diverter valve that lets hot water from the engine into a heater core, which is a device like a small radiator, which heats the air when it flows past. there is probably a second heater core for the rear. the valve for that one, or the switch or relay to control it is bad.
With your front heat working the most likely cause for the rear heat not to work is that there is an obstruction in the line that is preventing the warm coolant to get to the rear heater core or the rear heater core itself is plugged and not circulating. Another possible cause could be low coolant level where the system has enough coolant to provide the front system with enought warm coolant but not the rear system. Hope this helps
This problem is caused my the blend door actuator motor, the electric motor turns to allow the flap to open either to cool or heat and then through the vents. The problem has been either the motor is going bad or the actual motor stops working. There are small plastic gears that strip in the motor causing this. I have replaced several in the rear unit. The front one is under the dash next to the heater core. The dashboard must be removed. It is a big job.
Please rate me and let me know if this helped.
Air inlet door
Temperature blend door
Auxiliary mode door
The auxiliary A/C and heating system has the following features:
The auxiliary A/C and heating system is functional whenever the instrument panel mounted A/C control is in any position except OFF.
The auxiliary blower motor recirculates the air inside the vehicle. Outside air is not available to the auxiliary system.
Air is cooled and dehumidified by the auxiliary A/C evaporator core only if the instrument panel mounted A/C control is adjusted to a position that requires A/C compressor operation.
The temperature blend door and the auxiliary mode door are controlled by electric actuator motors.
Movement of the auxiliary temperature control between COOL and WARM causes a corresponding movement of the temperature blend door to mix air flowing through and around the auxiliary heater core.
Movement of the auxiliary mode control between PANEL and FLOOR causes a corresponding movement of the auxiliary mode door to direct air flow between the panel (headliner) registers and the floor duct.
I would say the auxiliary mode door (#4) electric motor is not wokring.
Auxiliary Blend Door Actuator —Air Distribution
Removal and Installation
Remove the RH quarter trim panel cup holder and access panel.
Remove the screws.
Disconnect the blend door actuator electrical connector.