Cal. Veh. Code § 27909
Any vehicle which carries liquefied petroleum gas fuel or natural gas, in a tank attached to a vehicle, in any concealed area, including trunks, compartments, or under the vehicle, shall display on the exterior of the vehicle the letters "CNG," "LNG," or "LPG," whichever type fuel is utilized, in block letters at least one inch high. The letters shall be of contrasting color and shall be placed as near as possible to the area of the location of the tank. Any vehicle fueled by liquefied petroleum gas fuel or by natural gas may also comply with this section by displaying on each side of the vehicle words or letters at least 0.25 inch high indicating that the vehicle is fueled by liquefied petroleum gas or natural gas. It is unlawful to dispense liquefied petroleum gas fuel or natural gas into any tank in a concealed area of any vehicle registered in California, unless the vehicle complies with the requirements of this section.
Ca. Veh. Code § 27909