42 U.S.C. § 7543 : US Code - Section 7543: State standards

    (a) Prohibition
      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.
    (b) Waiver
      (1) The Administrator shall, after notice and opportunity for
    public hearing, waive application of this section to any State
    which has adopted standards (other than crankcase emission
    standards) for the control of emissions from new motor vehicles or
    new motor vehicle engines prior to March 30, 1966, if the State
    determines that the State standards will be, in the aggregate, at
    least as protective of public health and welfare as applicable
    Federal standards. No such waiver shall be granted if the
    Administrator finds that - 
        (A) the determination of the State is arbitrary and capricious,
        (B) such State does not need such State standards to meet
      compelling and extraordinary conditions, or
        (C) such State standards and accompanying enforcement
      procedures are not consistent with section 7521(a) of this title.

      (2) 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).
      (3) 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.
    (c) Certification of vehicle parts or engine parts
      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) of
    this section.
    (d) Control, regulation, or restrictions on registered or licensed
      motor vehicles
      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.
    (e) Nonroad engines or vehicles
      (1) Prohibition on certain State standards
        No State or any political subdivision thereof shall adopt or
      attempt to enforce any standard or other requirement relating to
      the control of emissions from either of the following new nonroad
      engines or nonroad vehicles subject to regulation under this
      chapter - 
          (A) New engines which are used in construction equipment or
        vehicles or used in farm equipment or vehicles and which are
        smaller than 175 horsepower.
          (B) New locomotives or new engines used in locomotives.

      Subsection (b) of this section shall not apply for purposes of
      this paragraph.
      (2) Other nonroad engines or vehicles
        (A) In the case of any nonroad vehicles or engines other than
      those referred to in subparagraph (A) or (B) of paragraph (1),
      the Administrator shall, after notice and opportunity for public
      hearing, authorize California to adopt and enforce standards and
      other requirements relating to the control of emissions from such
      vehicles or engines if California determines that California
      standards will be, in the aggregate, at least as protective of
      public health and welfare as applicable Federal standards. No
      such authorization shall be granted if the Administrator finds
      that - 
          (i) the determination of California is arbitrary and
        capricious,
          (ii) California does not need such California standards to
        meet compelling and extraordinary conditions, or
          (iii) California standards and accompanying enforcement
        procedures are not consistent with this section.

        (B) Any State other than California which has plan provisions
      approved under part D of subchapter I of this chapter may adopt
      and enforce, after notice to the Administrator, for any period,
      standards relating to control of emissions from nonroad vehicles
      or engines (other than those referred to in subparagraph (A) or
      (B) of paragraph (1)) and take such other actions as are referred
      to in subparagraph (A) of this paragraph respecting such vehicles
      or engines if - 
          (i) such standards and implementation and enforcement are
        identical, for the period concerned, to the California
        standards authorized by the Administrator under subparagraph
        (A), and
          (ii) California and such State adopt such standards at least
        2 years before commencement of the period for which the
        standards take effect.

      The Administrator shall issue regulations to implement this
      subsection.