1) First, you must rule out health issues. The urine must be tested which means a trip to the vet. For a UTI, our cats have received antibiotics and pain meds (which helped them use the litterbox by helping them not feel pain.) I also gave the cats 3 ccs of water by mouth three times a day for the 14 days of meds. For idiopathic bladder inflammation, they were given pain meds and I gave them 3 ccs of water three times a day to help keep their bladders flush and working (not sure why it works but it does lead to progress.) Meanwhile, I let the sick cats use towels, empty boxes, plastic boxes for a litterbox and placed the objects near the litterboxes to encourage them to use the litterboxes or at least to continue to go near where the boxes were located (and not elsewhere in the house.) A cat who is sick will go anywhere especially when a painful bladder means they must eliminate NOW. As they got better (in a number of hours and days), the ill cats eventually returned to using the litterboxes.
2) Secondly, you must have clean litterboxes and many of them. Scoop out litterboxes 2-4 times a day. Wash the boxes weekly with soap and water. Replace old, scratched up boxes which might retain the scent of waste. Wipe them down each day because dust and old urine and such can get on the edges and sides and if a cat smells it, may not like it and may not use the box. Have 2 per cat and have one on each floor or areas of the house where the cat wants to go-as much as this is possible. We have 2 in each of the two bedrooms upstairs and 8 in the basement. None on the first floor because of space constraints.
3) Find what litter they like. As for the litter, cats like almost any litter but you need to test a few to see what YOUR cat likes. Ours do not like heavy scented litter but regular scented litter is just fine. It doesn't have to be expensive. But we do use Fresh Step, Cat Attract, and Wheat. They did not like corn or pine. Do not use liners. Do not add fresheners. However, maybe YOUR cat won't mind.
4) Discover if there is anything upsetting the cat. Is there another pet/cat/dog in the house that could be bullying or upsetting the cat? Or a person or people the cat may not like? Or work being done in the house or in the neighborhood that is creating noise? Is it being closed off from you during the day or evening? Is the cat alone during the day? Is the cat bored? Does the cat need more cat time with you, more playtime, more grooming, more attention? What happened in the days or weeks leading up to the first incident? Once you have discovered a possible cause for what is causing anxiety in your cat, you need to eliminate the cause or lessen its impact on the cat. The cat needs to remain calm and unthreatened for awhile before it learns how to handle stresses. If another cat or pet is a bully, you need to separate the two when you are not around. Give them each space and their own food bowls and sleeping areas and their own time with you. If the bully begins to attack or position itself in a threatening position (like hovering over or staring at the other cat) you'll need to remove the bully for a time out. If a person upsets the cat, take the cat out of the room when the person is around or try to get them to interact better with each other. Try having the person give treats or food to the cat. If the cat is bored when you are not around, put toys out or more dry food or treats around-on top of and underneath furniture and objects. Use cat nip scattered around in certain areas of the room/floor/house for the cat to find and roll around in and to eat. Is there noise that bothers the cat? If it is possible to move the cat to another part of the house away from the noise, then do so. Shut doors, windows, pull shades in order to block out much of the noise and the sight of the noise (for instance, if workers are working in or outside your home.) And yes, even cats in the yard might upset your cat and cause him stress. If possible, try to close the shades. Remember to keep the cat calm and happy as much as possible.
5) Do not reinforce negative reactions to stress by allowing him to remain where stress-causing issues remain. Do not reinforce his apprehension by petting him when he is upset about something. Try instead to change the subject-offer him a treat, toss a toy, remove him from the stress, brush him. If you pet him when he is upset, scientists say you are agreeing with him that he should be unhappy because there is a reason to be unhappy. In other words, his reaction to stress and unhappiness is now confirmed. And it is now a habit. His anxiety will be relieved by inappropriate behavior-namely going out of the box.
6) Next, determine where is the cat eliminating. Investigate and try to determine if there is a pattern to places or times the cat goes. A cat that is upset with anxiety may spray an object, wall, window, surface as well as use the floor, appliance, sink, counter, blanket, furniture, etc. upon which to eliminate. There will seem to be no rhyme or reason. The cat doesn't hate nor love the object it chooses. It doesn't hate you. Something has triggered it to choose a certain spot. Don't try to figure out the why. You might be able to eliminate the object of desire by removing it but you haven't removed the need to use an object. Let's concentrate or removing the need.
7) Clean, clean, and clean again to begin with. Clean wherever you smell urine. Clean the entire room until it is eliminated. Wash what can be washed. Use bleach on surfaces where possible. If you smell urine in a room, it might be there or it might be blowing in from another area or even the window. Urine tends to change smell as it ages. It goes from smelling like warm, fresh urine to something old, stinky, dirty, maybe even smelling like cat food or maple syrup or decaying trash. Take mild soap but not harsh cleansers or vinegar or ammonia based products. Go around the house with a bucket of soapy water and a few cloths and on your hands and knees, go from room to room, object to object and sniff around. Clean baseboards, and walls up 2-3 feet high. Mop the floors. Wipe down furniture with a damp cloth to see if a urine smell comes up. Clean the furniture if it does. Check your curtains/shades, even blinds. Check mirrors, windows, desktops, countertops, etc. But cats will rehit a spot because it's a cold, smooth spot; because the area brings on stress, because the smell is still there or is nearby and you've not yet found it to eliminate it.
8) Next, eliminate the smell, try to prevent the use of a favorite spot. If there is a spot that keeps getting hit, clean it with soap, then maybe with bleach if the surface will take it, then cover it with a hospital pad/bed liner-a package of 18 from Target is less than $6.00. Tape the liner up over the area if possible-like a wall, a corner, a desktop, etc. The cat will be dissuaded from going there, or might go on the pad which will be easier to clean up and easier to see where the urine is. For areas such as a brick fireplace, after thorough cleaning, douse it with baking soda and let sit for a week. Then vacuum it up. The smell should be gone and the cats shouldn't bother it again. If they do, repeat the process. Then think about covering it. Take away or move any object that is getting hit and maybe that will stop the cat, also.
9) Make the cat happy where it wants to eliminate. I have placed cat nip in certain areas when we had issues to dissuade the cat and make the area a pleasant place for the cat to hang out instead of going. Food also works. Feed the cat treats or dry food where it wants to eliminate-windows, appliances, counters, floors, etc. We have developed a routine where we give the cats treats in every room. The cats seems to associate the area with food and don't eliminate there.
10) Develop a routine for the cat for litterbox visits and make it a pleasant trip. Take the cat and cat treats to the litterboxes three or four times a day. This will eventually work but takes patience and time. Take the cat to the box, place cat gently in box, and when the cat comes out of box (which will be almost immediately)-whether it has gone or not-give the cat a treat. Begin to associate getting into the box with pleasure-food. Take the cat to the box in the morning when you first wake up, around lunch, late afternoon, evening, and before bedtime-however often you can. Spend 5 minutes near the boxes with the cat, talk to the cat, encourage the cat to use the box, pet the cat, etc. Don't yell or get angry and don't place the cat in the box more than once unless the cat gets into and out of the boxes on its own. Then do it one more time. Make sure things are calm and that other cats are not bullying or running around and that loud noise or music isn't playing. Routines will help establish when it needs to eliminate, will eventually help it associate the litterbox as a pleasant experience, and it will help you know how often and therefore when to expect the cat to need to go.
11) Felaway, the pheromone plug-ins and spray do seem to calm our cats but I can't say that it alone would work to solve out of the box issues. The spray works best when used in areas where the cat once sprayed or around areas it visits/sleeps often and not when sprayed all around the house like one would air freshener. We've actually had the cat urinate where we put Feliway because I think he thought that another cat was in the house. You don't have to spray it a lot to get it on an area. A gentle spritz will suffice. But never use it on the cat. It's not meant to be put on them and would sicken the cat. Felaway can be ordered for less than $25 from various on-line stores.
12) And finally, even while still doing all of the above, you should think about using meds to control the anxiety that might be the real cause of the litterbox problem. Without behavioral training, without proper litterbox maintenance, without trying to calm the cat and make it happy, meds alone will not solve your problem. Valium is a good anti-anxiety med and the only one heart patients can use. Amitriptyline might lead to urine retention, causing the cat not to eliminate but once a day (while the bladder noticeably balloons as it did for one of our cats.) But so far, Prozac, compounded into a liquid form and given at .120 a day (although we began dosing it at .10 and worked up over 8 months) is working out very well for one of our other cats as we continue to do treat training, timing his elimination needs, and continuously cleaning litterboxes.