A dog door can be hinged or spring operated or, more commonly, a "flap" which is a magnetic dog door installed into a door or wall to allow your dog to come and go at will without needing a person to open the door. Flaps hang from the top of the opening and flap when the dog passes through. Many pet owners find dog doors to be convenient and it reduces unwanted behavior from your dog such as scratching on doors or walls or relieving itself in the house.
Most dogs learn to use the dog door minutes after installing it. Using bits of meat or several toys for bait and the lesson is quick and easy, but some dogs find the whole experience frightening. The issues seem to be that the dog feels "trapped" inside when the flap is on top of him and the popping noise the magnets make when the door snaps closed. Luckily, there are steps you can take to train your dog to overcome these fears and begin to enjoy the freedom of coming and going at will.
Training should be spread out over several days and, if your dog is extremely anxious, a week or more may be needed. Don't expect this to happen overnight. Training will require leaving the dog door open or partially open so it is best to do it during a mild time of year or over several nice days.
° First, completely remove the flap covering from the insert. If the flap cannot be removed, tape the flap up and completely out of the way, using heavy packing or duct tape. Make sure it is 100% open and the dog can see outside. By using treats, or toys, bait your dog through the opening until he passes through it with no problems. If it is still too scary, try sitting on the opposite side of the door from your dog and using a high quality stinky treat like cheese or meat, coax your dog to just put his head through; or set the food inside the door opening and the dog outside and wait for the dog to get the courage to come through. When he does come through, praise him profusely and play with him for 5 to 15 minutes before trying the training step again. If you put the dog back out immediately, he may think coming in was a bad thing.
°With the flap still out of the way and the dog freely coming and going through the unobstructed opening, hang an old hand or dish towel that covers ¼ or ½ of the dog door opening. With the opening now partially obstructed, work with your dog by entering and exiting through it. Gradually move the towel down the opening until it covers more and more of the door. When your dog is successfully passing through the dog door as it is completely covered with the towel you are ready to move on.
°Replace the towel with a covering that is a little firmer and more like the dog doors actual flap. Ideally, your dog's experience when passing through this door will be identical to the experience of passing through the dog door minus the popping and snapping noises. Whether using plastic or flimsy cardboard, cut it to size so that it can swing back and forth through the door. Attach the covering so that it covers ¼ or ½ of the dog door opening. Once again, gradually lower the covering and when your dog is freely passing through the silent door covering, you are ready for the final step.
°Reattach or untape the dog door flap and coax your dog through. If you still have a problem, tape the bottom half or just a corner of the flap up so it shows daylight. Your dog has used the method successfully so he should readily pass through. After a day or two, your dog should be ready to use the dog door in the normal way so it is safe to untape it.