42 U.S.C. § 7543: State standards

No State or any political subdivision thereof shall adopt or attempt to enforce any standard relating to the control of emissions from new motor vehicles or new motor vehicle engines subject to this part. No State shall require certification, inspection, or any other approval relating to the control of emissions from any new motor vehicle or new motor vehicle engine as condition precedent to the initial retail sale, titling (if any), or registration of such motor vehicle, motor vehicle engine, or equipment. the determination of the State is arbitrary and capricious, such State does not need such State standards to meet compelling and extraordinary conditions, or such State standards and accompanying enforcement procedures are not consistent with section 7521(a) of this title . If each State standard is at least as stringent as the comparable applicable Federal standard, such State standard shall be deemed to be at least as protective of health and welfare as such Federal standards for purposes of paragraph (1). In the case of any new motor vehicle or new motor vehicle engine to which State standards apply pursuant to a waiver granted under paragraph (1), compliance with such State standards shall be treated as compliance with applicable Federal standards for purposes of this subchapter. Whenever a regulation with respect to any motor vehicle part or motor vehicle engine part is in effect under section 7541(a)(2) of this title , no State or political subdivision thereof shall adopt or attempt to enforce any standard or any requirement of certification, inspection, or approval which relates to motor vehicle emissions and is applicable to the same aspect of such part. The preceding sentence shall not apply in the case of a State with respect to which a waiver is in effect under subsection (b). Nothing in this part shall preclude or deny to any State or political subdivision thereof the right otherwise to control, regulate, or restrict the use, operation, or movement of registered or licensed motor vehicles. New engines which are used in construction equipment or vehicles or used in farm equipment or vehicles and which are smaller than 175 horsepower. New locomotives or new engines used in locomotives. the determination of California is arbitrary and capricious, California does not need such California standards to meet compelling and extraordinary conditions, or California standards and accompanying enforcement procedures are not consistent with this section. such standards and implementation and enforcement are identical, for the period concerned, to the California standards authorized by the Administrator under subparagraph (A), and California and such State adopt such standards at least 2 years before commencement of the period for which the standards take effect.