* Low Refrigerant: This is the most common culprit. While driving, the engine's higher RPMs may allow the compressor to generate enough pressure to circulate the insufficient refrigerant, providing some cooling to the front. However, at idle, the lower RPMs reduce the compressor's output, leading to insufficient pressure to cool the system effectively. The rear AC, being further from the compressor, would be the first to suffer from low pressure and therefore blow hot air regardless of settings.
* Compressor Clutch Cycling: If the compressor clutch is engaging and disengaging erratically (perhaps due to low refrigerant or a failing clutch), the system might work intermittently. At idle, the reduced pressure might cause the clutch to cycle less frequently, resulting in insufficient cooling.
* Restricted Expansion Valve or Orifice Tube (Front): A partially blocked expansion valve (or orifice tube in older systems) on the front AC system's low-pressure side could cause similar behavior. Adequate flow at higher RPMs, but insufficient flow at idle.
* Faulty Low-Pressure Switch: This switch is responsible for turning the compressor on and off based on pressure. A faulty switch could prevent the compressor from running at idle, leading to a lack of cooling.
* Condenser Issues (though less likely): A severely clogged condenser (the radiator-like component at the front) could restrict airflow and reduce efficiency, but this would likely affect both front and rear AC at all times, not just at idle.
* Blend Door Issues (Rear ONLY): While the hot rear air seems related to low-pressure issues, there's a small chance the blend door actuator (the motor that controls hot/cold air mixing in the rear) is faulty or stuck on hot. However, this wouldn't explain the front AC behavior.
Troubleshooting Steps:
1. Check Refrigerant Level: This is the first and easiest step. A qualified technician needs to use gauges to check the refrigerant pressure and quantity. Low refrigerant points towards a leak, which needs to be found and repaired before adding more refrigerant.
2. Inspect the Compressor Clutch: Watch the clutch while the engine is running. It should engage consistently. Erratic behavior suggests a problem with the clutch itself, the wiring, or low refrigerant pressure.
3. Examine the Condenser: Look for any debris or damage that might restrict airflow.
4. Check the Low-Pressure Switch: A technician can test this switch to ensure it's functioning correctly.
5. Diagnose Blend Door (Rear only): If the other steps don't resolve the rear AC issue, investigate the blend door actuator for proper operation.
Important Note: Don't attempt to add refrigerant yourself unless you have the proper equipment and experience. Improper handling of refrigerant can be dangerous. Take your Tahoe to a qualified AC technician for diagnosis and repair. They have the tools and knowledge to pinpoint the exact cause and fix it safely and efficiently.
