What Causes the Air Conditioning Not to Cool When the Car is Idling?

[MirandaAnn]

Here are the reasons why the air conditioning does not cool when idling: Refrigerant leakage in the air conditioning system: Refrigerant leakage leads to insufficient refrigerant, causing the air conditioning to fail to cool. The solution is to add refrigerant along with fluorescent dye. After the vehicle has been used for a period, inspect the air conditioning system with a specialized fluorescent lamp. If there is a leak, the leaking point will glow, and the leaking part or component should be repaired or replaced. Low engine speed: When the car is idling, the engine speed is low, causing the compressor speed to also decrease. Combined with insufficient refrigerant, the instantaneous pressure cannot reach the standard state, resulting in excessively low high pressure and the air conditioning not cooling. Observe through the sight glass on the dryer tank. During normal operation of the air conditioning, if continuous bubbles appear in the sight glass, it indicates insufficient refrigerant. If bubble flipping occurs, it means the refrigerant is severely insufficient and needs to be added. Damaged expansion valve: A damaged expansion valve prevents the system before the valve from forming high pressure, and the high-pressure refrigerant cannot reach 150 Kpa. As a result, the evaporator after the expansion valve cannot cool down, leading to ineffective air conditioning. Internal wear of the air conditioning compressor causes the pressure between the compressor and the expansion valve to fail to reach 150 Kpa to 200 Kpa, preventing the refrigerant in the system from circulating normally and causing the air conditioning to not cool. If severe internal wear of the air conditioning compressor is found, replacing the compressor will resolve the issue.

[McAshlyn]

I often encounter this situation during daily driving—the AC doesn't blow cold air when the car is idling, which is actually quite common. The first possible reason could be a refrigerant leak or insufficient refrigerant in the AC system, which prevents the compressor from functioning properly, especially at idle when the engine RPM is low and the compressor lacks sufficient power. Another possibility is that the condenser or cooling fan is clogged with dust, reducing heat dissipation efficiency at idle and significantly diminishing cooling performance. It could also be due to a dirty cabin air filter obstructing airflow or the compressor itself being worn out and failing. If this happens, I recommend first checking whether the cooling fan in the engine bay is running. If not, there might be an electrical issue or a faulty temperature sensor. The simplest method is to lightly press the accelerator to increase RPM and see if cooling resumes—if it does, then the problem is insufficient load at idle. On hot days, don’t take any chances; head to a repair shop immediately to check the refrigerant and the entire system to avoid safety risks on the road.