Why does my car AC only work when driving?
AC that's cold at highway speed but warm at idle or in traffic almost always means the AC condenser fan isn't kicking on. The condenser sits in front of the radiator and needs airflow to dump the heat the system is removing from your cabin — at speed the wind does it; at idle the electric fan has to. Look for a dead fan, a blown fan relay, or a bad refrigerant pressure switch that's supposed to trigger the fan.
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