California Code, Health and Safety Code - HSC § 43602

(a) 350 parts per million hydrocarbons.

(b) 2 percent carbon monoxide.

(c) 800 parts per million nitrogen oxide.

However, if no exhaust device meets all three of the maximums specified in subdivisions (a), (b), and (c), the state board may certify an exhaust device which meets any two of the three maximums specified, if the installation of such a device in a motor vehicle would not increase the other emission in excess of the emission of that pollutant by the vehicle in the absence of such a device.

If two or more exhaust devices are certified that they meet the requirements of this section, the state board may not require the installation of more than one exhaust device on any motor vehicle.