Cats with certain diseases (e.g. bladder infections, diarrhea, constipation) will avoid the litter box due to the discomfort associated with eliminating. Once the medical problems are successfully treated the cat may continue to eliminate away from the box due to the emergence of unacceptable surface or location preferences.
In some cases, you can break the habit and prevent further soiling by cleaning up the odor and changing the behavioral function of the soiled areas. You simply set the areas up so the pet uses them for behaviors that aren’t compatible with eliminating, such as feeding, play, resting, etc. Place food and water bowls, scratching posts, ‘kitty condos’, toys or bedding in the areas. You should also take the pet there for play, to receive treats, massages or to be groomed (assuming he likes to be groomed). After two to eight weeks, depending on the duration of the problem, you can begin removing food bowls and moving the the other objects back to their original places.