[Tech Note] - The A/C air coming out of the vents is colder on one side than the other.

This is an indication that the car's system is low on Freon. The best recommendation to get the best results is to evacuate the system, leak check , recharge with oil and dye mixture, and add new Freon.

Note: Due to the properties of R134a Freon, a simple top up of the system can cause overfilling and higher than normal pressures. Eventually the Freon charge ends up being blown out the pressure pop-off valve on the compressor and you are back where you started; plus the a/c will not cool as well as fresh charge from an evacuated system.