An enzyme cleaner definitely as the first port of call
Came here to say this.
One of our cats has ‘issues.’ I bought a Vax which lasted just over a year of very occasional use before starting to trip out the house electrics. Vax weren’t interested because I’d neglected to fill out the free extended warranty details on their website when I bought it. They offered me an unspecified discount on a new machine but I was similarly disinterested.
Anyway. As a carpet cleaner it (ho ho!) sucked. The best results I had for neutralising the stench of stale cat pish was soaking an old tea towel in a strong solution of Vanish and lying it over the affected area. The oxygen/enzyme-based cleaner kills whatever it is that causes the smell, and the towel leeches out the urine. Rinse and repeat for a few days.