There is definitely a problem when you observe that your car’s A/C is blowing warm air instead of cold air. It’s very frustrating especially during hot summer. One reason could be insufficient refrigerant due to some leakage.
But when you encounter this problem and you hear a very constrained hissing sound from the evaporator side (passenger side), then that’s a sign of a malfunctioning expansion valve. One effect also when you have malfunctioning expansion valve is that the engine idle speed drops significantly when the compressor motor kicks in.