How to clean heavily soiled carpet?

I inherited the same problem in our newest house. We were aware of the problem when we moved in. I bought my own vintage Steam Doctor machine off Ebay for repeated use. After deep shampooing the carpets about 6 times, I went to Petsmart and bought the special concentrate enzyme designed to neutralize the smell. It did not work. The best we could do was after deep shampooing, let the carpet dry completely and then apply baking soda based carpet deodorizer. The carpet looks clean and after a couple days the odor is gone. Each time the carpet is wet (for shampooing) the odor returns. The problem is the urine soaks completely through the carpet, backing and into the floorboards. Any new pet will smell the old urine, no matter how clean you get it, and designate that spot for potty time. We had to get rid of our chihuahua who never had this problem before moving here. The extreme solution is to rip out the carpet and pad and replace after deep scrubbing the bare floor.

You really need a professional clean as they have the equipment to reach down deep into the pile....carpet pile, even the padding behind whereas nothing you can rent or own as the suction power. I wonder if the pile itself is just degrading and loosing it's softness. Happening in my house even with pro cleaning and constant vacuuming.

I use either oxi clean or make my own with vinager dish soap and peroxide. Works great