1. Simple Checks:
* Rear AC Controls: Make sure the rear AC is actually turned ON and the fan speed is set above zero. Check all rear climate control settings (temperature, fan speed, mode).
* Power Supply (Fuses): Check the fuses related to the rear blower motor. Your owner's manual will show the fuse box locations and diagrams. Pay attention to fuses labeled "rear blower," "climate control," or similar. Replace any blown fuses.
* Power Supply (Relays): Similar to fuses, relays can fail. These are typically electromagnetic switches. While more difficult to check visually, you might hear a click from a relay when activating the rear AC. A failing relay may need replacement.
* Rear Blower Motor Switch (if applicable): Some Yukon models may have a separate switch to turn the rear blower on/off. Ensure this switch is in the correct position.
2. More Involved Checks (requiring some mechanical knowledge):
* Blower Motor Resistor: This component controls the fan speeds. A faulty resistor will often result in no fan operation at any speed or only working on the highest speed. Accessing this will likely require removing some interior panels.
* Blower Motor: The motor itself could have failed. Testing this requires multimeter skills to check voltage and resistance. Replacement is typically a more involved repair.
* Wiring and Connections: Check the wiring harness and connectors leading to the blower motor and other related components for any damage, corrosion, or loose connections. This often requires removing interior trim.
* Control Module (HVAC): The rear climate control module could be faulty. This is a complex component, and diagnosis often requires professional tools and expertise.
3. Other Potential Issues (less likely but possible):
* Low Refrigerant: While it won't directly affect the blower motor, low refrigerant could cause the rear AC system to not activate the blower even when the controls are set.
* Blend Door Actuators: These control the direction of air flow. A faulty actuator could prevent air from reaching the rear vents even if the blower is working.
* Vacuum Leaks: Some older systems rely on vacuum to control air flow. Leaks in the vacuum lines can disrupt proper operation.
Recommendations:
* Consult your owner's manual: It provides crucial information on fuse locations, component diagrams, and troubleshooting steps specific to your GMC Yukon's year and model.
* Start with the simplest checks: Fuses and the rear climate control settings are the easiest to check first.
* If you're not comfortable with electrical or mechanical work: Take your Yukon to a qualified mechanic or an authorized GMC dealership for diagnosis and repair.
Attempting repairs beyond your skill level can lead to further damage. Safety first! If you are unsure about any step, seek professional assistance.
