Why does my car’s AC blow warm at idle but cool when driving?
At idle, two things fall: airflow through the condenser and compressor speed. A healthy system copes; a system with a weak point shows it first at idle in high ambient temperatures. The weak point can be in several places, so this is a symptom to test, not a part to guess.
How the system rejects heat
The compressor pumps refrigerant to high pressure and temperature. In the condenser, in front of the radiator, it rejects heat to outside air and condenses into liquid. The expansion valve (or orifice) drops its pressure, and in the evaporator behind the dashboard it boils and absorbs heat from the cabin air. If the condenser cannot reject enough heat, high-side pressure and temperature rise and cooling falls.
Why idle exposes problems
When driving, ram air flows through the condenser; at idle, only the electric fans move air. In very hot ambient air, the margin is small. Possible contributors include: a fan not running or running slowly; a condenser blocked by sand, insects or debris, or with damaged fins; an incorrect refrigerant charge, too low or too high; an expansion valve not regulating correctly; a compressor or its control valve (on variable-displacement compressors) not building enough flow at low speed; air or moisture in the system; or engine cooling issues heating the same airflow.
How it is checked
High- and low-side pressures read against ambient temperature, vent temperature, fan operation and condenser condition, then a leak test (UV dye and electronic detector) if the charge is in question. Only then is a component identified. The same symptom has led to unnecessary compressors more often than any other AC fault.