Which symptom indicates that the air conditioning system may have low refrigerant?

The symptom indicating that the air conditioning system may have low refrigerant is when the AC is blowing warm air. This occurs because refrigerant is essential for the cooling process in an air conditioning system. When the refrigerant levels are low, there is not enough material to absorb heat from the cabin air, leading to insufficient cooling. As a result, the air coming from the AC vents feels warm rather than cool, signaling a potential refrigerant leak or depletion.

In contrast, other symptoms mentioned, such as a spongy brake pedal, clunking noises, or rough engine operation, are not directly related to the performance of the air conditioning system. A spongy brake pedal typically indicates issues within the braking system, such as air in the brake lines or worn brake components. Clunking noises could point to suspension problems or other mechanical issues, and rough engine operation likely suggests fuel or ignition issues. Therefore, warm air from the AC is a definitive sign to investigate refrigerant levels in the cooling system.