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Try to back wash the heater core first if it doesnt work then its clogged up try to turn the heater on the front with the heater off for the back keep an eye on the temp guage if it goes up than normal then its clogged....
Fyi u have to run the engine for awhile with the heater fan on
There is a rear control module and a rear data processor for the blower motor and the blend doors. Your probably going to need a shop manual to have the test procedures for those circuits. One orange wire goes to the module from a 10amp fuse in the box in the engine compartment, the other orange wire goes to the module from a 30amp fuse in the same box. There is a third orange wire between the front HVAC control panel and the rear module.
Rear heat not working ? The blower motor isn't working ? No air coming from the rear vent's ? Is it just a yukon or XL or denali ? What size engine ? 4.8 , 5.3 or 6.0 l Manual or automatic climate control ? Does the blower motor work on any speed ? The front an rear heater system are seperate , have there own electrical circuits . Could have a couple of different systems ,could have a control processor or relay's an a resistor block . Do you know how to test automotive electrical circuit's using a wiring diagram and a DVOM - digital volt ohm meter ? Plus to get at the rear heater-A/C unit you would need to take the right rear trim panel off . 2003 Tahoe No Rear Heat Fix Suburban Yukon XL Repair
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.
{: ) If the front a/c and heater is working fine, that's good. At least the problem can be isolated on the rear HVAC circuits only.
Check first the RR HVAC fuse 30amp at the underhood fuseblock and the IGN 3 fuse 10amp at the IP fuseblock. If you found them good, then you can begin diagnosing the blower circuits wirings and connectors.
A faulty blower definitely will not give any output. If it doesn't spin for some reason, rear a/c or heating cannot be circulated.
A faulty blower resistor or speed control processor can also cause a zero air output. Sometimes, the blower may only work at certain speeds only.
A faulty rear HVAC control switch may not give the proper signals to the blower resistor or speed controller, and in turn will render the blower motor disabled.
Sometimes it can be a terminal connector being loose or burnt. However, the most usual cause for this type of problem is a faulty blower resistor.
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