Sam Mouzer, a survivalist featured on the Discovery series, tells TMZ ... his penis is fully working again after his grueling -- and super cringe to watch -- medical emergency.

TMZ broke the story ... Sam's burning moment was captured as he filmed his episode for the new season -- an ember popped from the campfire, scorched his family pride and prompted him to scream out in pain.

Sam tells us he and fellow contestant, Lilly, were getting peppered with hot coals that fateful night in New Mexico ... and it was only a matter of time before one landed in the wrong spot.

Bullseye!!! He says it actually felt like putting out a cigarette on bare skin.

Sam says the pain was sudden and sharp, then the discomfort deepened and his penis started throbbing. Heard enough?

The good news ... Sam's back home in the United Kingdom after surviving 21 days in the Wild West, and he's having sex with his wife again. He also says the wound didn't get infected, so he's got a special 'thank you' for the show's medical staff.

Sam says he did come home with a little souvenir ... a very small blemish on the tip, which he refers to as his "Naked And Afraid" tattoo. Sounds like lots of contestants have them, he was just unlucky enough to get his in a very delicate area.