Why does hot air blow only on the driver's side but not on the passengers?

Your vehicle is equipped with a “dual zone” climate control system, which allows you to have a different temperature, if desired, in the passenger side zone versus the driver’s side zone of the vehicle. When one side will not adjust to the proper temperature, there is either a problem with the control mechanism, the motor that turns the doors behind the dash inside the ventilation system plenum which meters the temperature from hot to cold, or a problem with the actual doors or plenum. Testing will need to be performed to find out what has failed and how it can be repaired.