Pomona police received a call about an injured individual at 292 E. Olive Street at around 6:45 p.m. Tuesday.
When officers arrived, they found a citizen attempting life-saving measures on an unresponsive 45-year-old man.
The victim had volunteered to assist the owner of a car to locate an oil leak, police said. Although the car was only supported by a small car jack, the victim insisted on crawling underneath the vehicle.
Police said the victim may have struck the car jack, causing it to malfunction and allowing the car to collapse on top of him.
The car's owner and a passerby lifted the vehicle and pulled the victim out, but the victim did not recover. He was pronounced dead at the scene by paramedics.