If you know where the urine spot(s) are you could try Simple Solutions Dog stain and odour remover. It's available from pets and home and amazon. It has enzymes in it that break down the urine. We've used it since Ddog was a pup and find it really effective. If it's for a large area I know Vax do a similar product for their machine and no doubt other brands to do. It really depends how much urine has been on the carpet though. We once had to rip a carpet out as no amount of cleaning was going to get rid of the smell.
We are going to change the carpet in the next few months, so I just need a short term solution. I’ll try Simple Solution. Do you just spray it on and leave it, or do I need to track down each area and rub it in? There are no visible stains.