According to the California Department of Transportation, the minimum fine for misuse of the carpool lane, also known as the "HOV" or "High Occupancy Vehicle" lane, is $481 as of 2014. Although no maximum fine is listed, it's suggested that most fines will be at or near the minimum amount.
The California Department of Transportation states the purpose of carpool lanes is to move as many people in as few cars as possible. Each vehicle that travels in a carpool lane must carry the minimum number of people posted at the freeway entrance signs, typically at least two, but sometimes three people. While children count as occupants, pets and unborn children in the womb do not.