They don't call this a miracle for nothing: Nature's Miracle Just for Cats is the only product I've ever tried that really truly works to remove the odor from a cat's decision not to use her litter box. Usually, the cat can  still smell it even if we can't, and the cat returns to the same spot again. But this stuff really seems to eliminate the odor even for the cat's nose. I put the Amazon page link here, but you can buy this anywhere pet products are sold. They have one for "pets" which I guess is great for dogs, but if you have a cat, get the one that says it's for cats. Buy the biggest size you can, because you need to use a lot for each accident (or in my cat's case, for each on-purpose) but it's well worth the money.