Carpet dog pee stains, advice please!

Does anyone have tips for older stubborn dog pee stains on carpet? I'm at a loss. I've tried multiple sprays etc and for some reason it will look like they've lightened up, but a few days go by and it dries even darker!! Please help! Our dog wasn't completely trained when we got him (he's trained now), but we rent and I want to get them out. Would steam cleaning work? Even our regular carpet cleaner doesn't work for long term disappearance.