If your vehicle is showing dual-zone climate blows hot on one side and cold on the other, here are the most likely causes and the parts you'll need to fix each one.
A slightly low refrigerant level causes only half of the A/C evaporator to get cold. This starves one side of the dual-zone system of cold air, making it blow warm while the other side blows cold.
Dual-zone systems use separate electric motors to mix hot and cold air for each side. If one actuator's internal plastic gears strip or the motor dies, that side gets stuck on a single temperature.
Disconnecting or replacing the battery can cause the climate control module to forget the blend door positions. The system requires a recalibration or relearn procedure to sync the doors again.
If the symptom happens primarily when the heater is on, a partially blocked heater core will only provide heat to one side of the ventilation box, leaving the other side blowing cold air.
The main climate control panel or module can suffer an internal electronic failure, preventing it from sending the correct temperature commands to one side's actuator.