One of the most common reasons healthy cats eliminate outside the litterbox is because they find some other place or surface more attractive. An aversion to the litterbox can arise for many reasons: The box may not be clean enough. It may be too close to the cat's food, water or large appliances that make frightening noises. Or other cats or dogs may harass the cat while it is in the box. Some cats may find a covered box too confining or be bothered by pan liners. Many cats are particular about the type of litter provided, and you may need to experiment to find a litter your cat is comfortable using. A preference study conducted several years ago by Peter Borchelt, Ph.D., a certified animal behaviorist in New York City, showed that most cats tend to prefer fine-grained sandy litters that are not dusty or heavily scented.
The key to solving problem elimination is to make the litterbox more attractive while making areas where the cat is house soiling unattractive. Sometimes, just cleaning the litterbox more frequently or changing its location while keeping the cat away from soiled areas may do the trick. Other cases may require careful observation of the cat's behavior, then systematic experimentation with the box or its contents to find the most attractive combination.
Cats can usually be kept off a soiled area by using a scent the cat finds offensive, such as a strong citrus odor, Ben-Gay or a strong floral room deodorizer; a mat that delivers a harmless static shock, such as a Scat Mat; or a device that produces a loud noise when the cat comes near. Of course, soiled spots should be thoroughly cleaned with a good cleaner recommended by your veterinarian to remove any scent that may attract the cat back to the area.