My 2004 Envoy XL rear will not work. The blower works and cold air comes out but no heat with the front or rear controllers. I suspect it is the blower resister. Where is the blower resister located and can this be the cause of the problem?
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I would start with the rear blower motor resistor, the resistor is what controls the speed of the fan, the resistor is located in the rear heater box near the blower, you will need to remove the right rear side panel to access the heater box. start with that
There is a blower motor control processor for the back blower - it takes input from the front control and the rear control and decides the fan speed. That processor could be faulty, or the blower motor, or a 30amp fuse in the engine compartment. In the front system you have a controller and blend doors that direct air thru the heater core and evaporator. You would need to check the operation of those blend doors.
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
If the front is still blowing air but not air conditioned air than it is likely that a flap that controls airflow to the front has failed. The rear would still work as it is controlled by an auxiliary control.
If there is no air coming out of the front at all I would suspect a blower motor as the rear has it's own blower.
Either way a scan tool that is able to access the HVAC system is going to be the only to accurately diagnose this problem.