"My heart told me to stop," Rivera said.
He found a man bleeding in the mangled, upside-down car.
"Someone else arrived and we both pulled him out and put him on the side of the road," Rivera said.
The injured man said there were children in the backseat, so Rivera searched, but found nothing. That's when he realized the man he just saved had pulled a fast one.
"He said, 'Hey, I think he's taking your car!' And it was the injured man taking it. And he left in my car," Rivera said. "It seems like he was drinking. He smelled a lot like liquor or beer."
Police say if you see an accident, the right thing to do is what Rivera did: safely stop and see if you can help. But no one expects what happened to Rivera.
"It makes me sad that I helped him and he still took my car," he said.
The Adams County Sheriff's Office is looking for Rivera's 1994 black Toyota Corolla. It has Colorado license plates 047-YBV. If you see the car, you're asked to call 911.
In the mean time, the man who fixes cars, doesn't have one himself.
"I do need the car because that's the car that I use to go to work," Rivera said. "What's really important to me are the tools that I had inside."
Left without his tools and wheels, Rivera also doesn't have any anger.
"Thank God he didn't hit me or anything and I'm fine," he said.
He says he doesn't regret stopping to help and he'd do it again. He also says he's glad he was the one targeted and not the other person who stopped to help.
"It was better that he took my car and not the other car because there were kids in it," he said.
Insurance won't cover the loss, but Rivera says he'll find a way to get to work - too many others are depending on him to get them back on the road.