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Without knowing the make and model of your car, this is a general answer. Check for a vacuum hose that has fallen off the engine. Sometimes, the small diameter plastic hoses develop cracks that are hard to see. If you find one that has melted from excessive heat you can repair them easily by taking a very sharp knife to score the diameter then bend the end to snap off the damaged part. Cutting it with wire cutters will only crush it and cause more cracks. Once you have prepared the ends of the tube use a rubber vacuum hose section to join the plastic tubes together.
Sounds like you have a vacuum leak to the control head. When you lose vacuum the airflow defaults to defrost. Check the vacuum hoses coming into the car on the passenger side under the dash. Supply vacuum is the black tube. There are five colors. Blue, red, yellow, white, and black.
This usually is related to the vacuum reservior tank/cannister or the hose leading to that chamber. Any other vacuum hose leak could cause a similar problem depending upon it's location.
The hose leading to the cannister is usually just a thin single hose/tube leading to the cannister. The system allows vacuum to build up in the vehicle and maintains vacuum on the A/C system under heavy acceleration where a loss of vacuum would cause the A/C to change operation (switching from defrost to other venting and vice-versa) due to the loss.
You could have a cracked vacuum hose, or a cracked cannister.
You can use regular carburetor cleaner in a spray can with a nozzle, and carefully spray the vacuum lines on your vehicle when idling, and should notice a change in RPMs if you spray near a leak. Only spray a small amount at a time and let the engine compartment air out between spraying/testing since this is a flammable product.
Defrost is a default position for when there is a loss of vacuum. The vacuum controls the blend air doors. Start at the intake manifold and follow the vacuum hoses. There should be one that goes to a resivour (I think its behind a fender). It should T off and go inside the car. If the resivour is cracked or any of the lines are leaking that will cause the blower to only go through defrost.
I have also seen where ford uses plastic tubes and the dry battery acid had fallen on the tubes and ate holes in them. If any of the plastic tubes are broken, then each broken end can be inserted into a small enough rubber vacuum hose to repair them.
The air controls in the heat/AC system are vacuum operated. When the controls don't cause the air flow to shift it is normally caused by lack of vacuum to the air flow system. This is normally caused by a break in a vacuum or a vacuum line that has come loose from a connection, usually under the hood. These vacuum lines are small black plastic tubes about 1/8 to 3/16 in diameter. My experience has been that places under the hood where these lines are exposed to high heat causes them to get brittle and break or crack. I have found lines that are between the battery and fire wall that don't get much air flow and are heated by the battery are a likely place to look. Look for the line that goes through the fire wall. that is likely the damaged line. Trace it back to find the crack or break. If you find this is the cause, replace all the section of line that is brittle. If you don't do that you will have the same problem again very soon. You can splice sections of this tubing with rubber hose of the proper inside diameter. Good luck.
if you mean the pipe at the froint of the engine under the inlet manifold then no, because when this one splits and sucks in air it races ,that is if its a europeon focus and not an american californian model