The CHP has a policy to enforce the license and registration laws with great zeal, this may have something to do with the fact the agency receives nearly all of its funding from the DMV fees collected through vehicle registration and license plates. California law issues two license plates to all cars and requires that a plate be on both the front and back of any car registered in this state. With that being said, what about the cars that come in from out of state?
Many states do not require a front license plate, in fact, the states only issue one plate which is to be displayed in the rear of the car. For example, Florida and South Carolina both do not compel an owner of a non-commercial vehicle to place a license plate on the front of the car. With this in mind, what if a cop stops a person driving a vehicle from one of these one-plate states solely for the reason that they were not displaying a front plate? California specifically addresses this issue by dictation that the car is not required to have a front plate if it is registered in that state. A car stop that eventually leads to a DUI arrest then that was based solely on the perceived violation of the law would presumably be illegal.
Indeed, this was the decision in a case called Reyes, where the appeals court ruled that the police are presumed to know the law, when they stop a driver and they are mistaken about what the law states, the stop is generally illegal. In that case, the officer found drugs while talking to the driver, the Court threw out the criminal charges saying that the officer had no reasonable basis to pull the driver over in the first place because his car was from Florida and the law in that jurisdiction only required one plate.
In cases where the vehicle is registered in CA however, the police can and will likely pull you over and the stop would be legal if the car did not display a front California license plate.