The incident happened Wednesday night at the intersection of W. Orangethorpe and Courtney avenues.
The Fullerton Police Department said the police cruiser had its emergency lights on, and several flares were placed on the road when a blue Tesla crashed into the vehicle.
"The driver of this Tesla admitted to engaging his vehicle's 'self-drive' mode while using his cell phone, a clear violation of responsible driving practices and California law," said police a news release.
No major injuries were reported. The Tesla driver is cooperating with the investigation, according to police.
Meanwhile, the deadly crash that was initially being investigated involved a motorcyclist.
Police said a driver hit the motorcyclist and one person was killed though further details weren't immediately available.