CA Health & Safety Code Section 43656

The state board may exempt from any schedule of installation adopted pursuant to Section 43654 or 43655:

(a)

Motor vehicles or classifications or subclassifications of motor vehicles for which certified devices are not available.

(b)

Motor vehicles or classifications or subclassifications of motor vehicles whose emissions are found by appropriate tests to meet applicable emission standards without an additional motor vehicle pollution control device.

(c)

Implements of husbandry.

(d)

Vehicles which qualify for special identification plates pursuant to Section 5004 of the Vehicle Code.