Drivers Side Blows Cold, Passenger Warm - Or Vise Versa
Most of today's cars and trucks have the ability to blow different air temperatures form the drivers and passenger side. Even if you do not have dual-zone controls, there still may be two separate doors and motors to control the side separately.
If all the basic A/C and heater systems are functioning properly, then a problem with one of the temp doors is most likely the cause.
Normally the temperature door motor is the failure. These motors control the door. Depending on which side has the problem, the replacement is different for each make and model. Need to refer to a service manual for the door and motor locations.