California Code, Health and Safety Code - HSC § 44253

If the Environmental Protection Agency adopts a system that provides for the assignment of one or more index numbers to vehicles in a manner, and for purposes, similar to the smog index provided for in this chapter, the state board shall adopt the index numbers assigned to vehicles by the Environmental Protection Agency as the smog index, but shall also specify a smog index consistent with the federal index system for any vehicle subject to this chapter for which the Environmental Protection Agency has not specified an index number.