I'm kind of going through the same thing right now with my cat. I can tell you what my vet recommended and I guess it works for some cats but hasn't so much with mine. It's worth a shot, though. My vet told me to try Feliway. It's something you stick in your outlet that's supposed to send out something in the air that calms the cats and helps with urine marking. I tried it and it didn't work for me but my vet said it worked with her cat. She also recommended "Kitty Prozak" for my cat and said it could help with the peeing because she thinks she's a stressed out kitty. Crazy, huh? But I'm trying it. So far, no luck. We also bought the "Natures Miracle" from Petsmart. It's a pet enzyme that's supposed to help with the smell. You clean the area and then spray this on. There's also something called "Urine B Gone" which I heard of but haven't tried yet. Rent a carpet cleaner and get the cleaning solution that has the pet enzyme in it to help neutralize the urine odor.
You're right, cat pee smell is awful. I'm about sick because our cat has practically ruined our carpet now in our downstairs finished basement and it was fairly new, nice carpet. We can't really afford to replace it at this point but it will have to be replaced, I'm sure.
At least your cat is quite a bit older than mine. 17 is a pretty good age for a cat (mine's almost 11). I don't like the idea of having to put mine down but I can't continue to live like this either. I will go as long as I can. I hope some of my suggestions work for you. I wish you the best. I hope it all works out for you. God bless.
Thanks so much for the suggestions. We tried all those cleaning supplies - LOL. Thankfully, my mom has tile...it's just the couch that my cat pees all over. Which is sick because of course we all sit on the couch. I might ask my vet about the Kitty Prozac! Or look for the Feliway! Thanks again so much! And good luck with your kitty, too!