How to address pet urine smell

[Joy Remington]

Several months ago when my HVAC contractor showed up to take care of an issue at one of my rentals, he told me that the dog urine smell in the house was so strong he was afraid he might pass out. The tenant has been in the house for almost 4 years. He always pays his rent on time and I have had no issues with him. How do I let him know that he needs to take care of the urine smell? Do I tell him that it was my contractor that noticed it? Do I do an inspection of the house and then "notice" it? If so, what kind of inspection would I be doing? I have not done inspections, though I know some of you say that it's extremely important. Any other suggestions on how to address the smell would be appreciated. Should I just send a letter? Etc?

Thanks, Joy