TMZ has confirmed Ronnie's autopsy results, which say the 62-year-old actor died due to "complications of metastatic colon carcinoma." The coroner says the cancer was stage 4.

The report says Ronnie only struggled with cancer for 3 weeks before his death.

It's also determined Ronnie battled atherosclerotic cardiovascular disease at the time of death -- which is a buildup of fats and cholesterol in the arteries.

TMZ broke the story ... law enforcement got a 911 call Thursday morning from a person who said Ronnie was outside their home and having a hard time breathing. A few minutes later, he stopped breathing.

Paramedics rushed to the San Fernando Valley address -- where bystanders were attempting CPR, but those efforts didn't work and he was pronounced dead at the scene.