I use “Complete” brand for Cats… Stain & Odor remover. I don’t remember where I got it but here’s the info on the back of the bottle:
Eight In One Pet Products
distributed by United Pet Group, Inc.
Cincinatti, Ohio 45255
(800) 645–5154
www.eightinonepet.com
There was another one I had once I think by Arm & Hammer but I can’t find it anymore. I got it in a tiny podunk pet store where I used to live. I don’t really ever have to use it so I forget where I buy the stuff and exactly what it was… lol. Anyway..
The problem right now is the possibility of the cat urine having gone through the carpet to the floor. You might not be able to get the smell out ever without ripping up the carpet and scrubbing the floorboards underneath, unfortunately. But this stuff I mentioned works really really well. I would mix white vinegar with water, half and half, and pour it generously (but not so it floods the place obviously) all over the spot on the carpet and let it seep down. Normally I wouldn’t suggest that but since you can’t rip up the carpet and get to the floorboards, it’s necessary. Let it sit for maybe 15 minutes. Then soak it all up using paper towels… get as much out of the carpet as you can. Then spray generously the spray I suggested (or whatever one you happen to get) and scrub the crap out of it. Make sure you test it out of sight first so you know it won’t fade the carpet. Let the spray sit another 10 minutes… then blot all the liquid out with paper towels again. Get yourself one of those carpet deodorizers for pets. Sprinkle it on the carpet, let it sit for 5 minutes, and vacuum it up. You should be good to go by then.