2003 1500 ram, floor heat and vent heat works, wont go to defrost

Some cars use vacuum from the engine to open and close the doors that direct the air to different positions. It is possible that the vacuum line to the controls is cracked or disconnected.

out of reffrigirant ,or leaking vacuum switch so when you put on defrost it won't actually work

Mode Door 1.
a.This door regulates air flow to either the dash vents or defrost/floor. If you have air flowing only through the vents with no defrost or floor, this door is failing. From our experience, this door is usually the first domino to fail. Not sure why, just is.
<!--[if !supportLists]-->4.Mode Door 2.
a.When mode door 1 is set to divert air away from the vents and into the second stage of the system, door #2 chooses whether air goes to the defrost vents to the windshield or out through the floor vents in the passenger and driver's side footwells. Failure is indicated by an inability to choose between defrost and floor.
b.This door turns out to be the most difficult to replace. In order to replace the door, the dash panel has to be removed. It's still better than the dealer fix because you don't have to evacuate the AC system, drain the radiator, and remove the plenum box. But it is more work than the other fixes. When this door breaks, it falls in the down position, blocking the floor vents and diverting all air through the defrost vents. From a safety standpoint, this is good since you'd most likely rather have a clear windshield than warm feet. We suspect that a large percentage of customers will just leave this door unrepaired. You will have heat through the dash vents and to the windshield, just nothing to the floor. It's inconvenient, but you'll have to make the determination if warm feet are worth the extra work to fix the door. An extra pair of socks is easier.

Check the vacume pipes under the viper shield behind the engine.

sounds like your cables in the selector switches may be loose or blocked

check the thin rubber pipes first that open the flaps in case its come off the valve or the lever arms been knocked off ,something stupid.failing which if it appears all ok then the clima computer might have had a brainstorm and then its the dealer.best of british with it anyway.

OLD BEHIND CAR

common problem, check under dash by the gas pedal if control arms are muving went selecting from ac to heat cuz this it is the control for the vents to go from ac to heat and to defrost.

you need to replace the panel...

Had the same problem. believe it or not i changed the gas cap and got heat all around. something about pressure.