The 53-year-old was driving a compact car which sped out of control in a westside neighbourhood of Los Angeles late on Friday morning, before ploughing into a house and bursting into flames, according to police.
No one inside the house was hurt, but the impact set the dwelling ablaze, requiring a response by dozens of firefighters.
A Los Angeles Police Department spokesperson said on Monday the cause and circumstances of the crash remain under investigation.
Heche is in a coma and has not regained consciousness since shortly after the accident, said Michael McConnell, a member of the Los Angeles talent management company representing her, Zero Gravity Management.
"At this time, she is in extreme critical condition," he told Reuters in a text message, adding that Heche "has significant pulmonary injury requiring mechanical ventilation and burns that require surgical intervention."
The Los Angeles Times quoted a Venice Beach salon owner, Richard Glass, recounting a visit by Heche to his shop shortly before the accident, describing her as "a sweet little girl" as she purchased a red wig on Friday morning.
Heche came to prominence for her Emmy-winning work on the daytime television drama Another World and went on to star in other screen roles including the HBO series Hung and such films as Wag the Dog and Cedar Rapids.
She made tabloid headlines in the late 1990s for an affair with comedian Ellen DeGeneres around the time that DeGeneres came out publicly as a lesbian.
Following their split, Heche wed cameraman Coleman Laffoon, but they later divorced, and she spent some years after that in a relationship with actor James Tupper, her co-star in the short-lived TV show Men in Trees.
