When manual climate control of my 2008 Elantra is switched to defrost mode the A/C goes on. .Why is this happening?

Pull the control panel out of the dash and see if the mode control vacuum switch has a good vacuum source or the switch itself leaks vacuum. The mode control is what moves the doors that diverts the air flow around to the vents, defrost and floor.

may be a factory setting do u use all of the controls on the panel for air, etc?

check HVAC fuse.if fuse okay heater control module could be the problem.

climate control is control by a controller or computer which read inside car temperature sensor and ambient sensor.best have vechicle check at garage you need diagnostic equipment to trouble shoot climate control.many things cause climate control problems. could have faulty temperature blend door actuator or bad sensor.

Unfortunately, it sounds like your HVAC control module is failing. You can try a reset by removing the negative battery lead (under the rear seat) for about 10 minutes, and then reconnecting it, but if that doesn't work, you will probably have to replace the module.

It all depends on whether your truck is equipped with Automatic Climate Control or Manual HVAC. You could have a broken control head assembly a broken mode door cable, a failed electronic mode door actuator, a broken mode door, or a loose or broken wire or connector.

To put it simply, your mode doors are not swithching modes - the reason for this can vary depending on which system you have.

MAKE SURE BLOWER MOTOR OPERATING IF NOT CHECK FUSE AND RELAY.ALSO CHECK HVAC FUSE THIS CONTROL HEATER + AC CONTROL SWITCH AND MODE SELECTION FOR OPENING AND CLOSING HEATER DOOR OR HEATER CONTROL VALVE AND AC+ DEFROST AIR DOOR.IF BLOWER AND HVAC FUSE OKAY.IF YOU HAVE MANUAL HEAT CONTROL MAKE SURE MODE SWITCH IS RECEIVING VACUUM. YOU HAVE TO REMOVE HEATER CONTROL SWITCH AND CHECK TO SEE IF GETTING VACUUM GETTING TO SWITCH LOOK FOR BROKE DISCONNECTED VACUUM LINES IN THE DASH AREA.IF YOU HAVE CLIMATE CONTROL.YOU COULD HAVE A FAULTY ELECTRICAL HEAT DOOR ACTUATOR.SOME CLIMATE CONTROL CARS HAS THE CLIMATE CONTROL PROGRAMMER HAS TO BE REPROGRAM BEFORE THE HEATER AND AC WILL WORK AGAIN.YOU NEED A DEALERSHIP SCANNER TO CHECK IT OUT.BUT IF YOU HAVE MANUAL CONTROL CHECK TO SEE IF YOU GETTING VACUUM.IF YOU GETTING VACUUM AT HEATER DOOR VACUUM ACTUATOR OR HEATER CONTROL VALVE EITHER ONE OF THOSE COMPONENTS COULD BE FAULTY.WARNING YOUR VECHICLE HAS AIRBAGS YOU HAVE TO DISARM THEM BY REMOVING FUSES WAIT 20 MINUTES BEFORE WORKING ON DASH AREA.YOU HAVE TO CRANK CAR UP TO CHECK TO SEE IF YOUR HVAC GETTING VACUUM.

If your Maxima is equipped with Automatic climate control, you must put your control unit into self-diagnostic mode, then follow the troubleshooting chart for whichever fault code is output by the climate control unit. (most likely a mode control motor fault)

If your Maxima is equipped with manual climate control, there are two main causes. The mode control (push button) switch unit and the mode control motor.
The switch must be tested first, then the wiring between the switch and the motor. If these things check good, then the motor should be replaced.

There is a climate control module behind dash on passenger side probably going to be the problem

Here's the wiring diagram of the manual AC controls. I suspect the problem is in the 'mode' switch/motor.