Trump is revoking California's Clean Air Act exemption. Does this automatically affect the 13 other states that use California's standards?

Question

President Trump is reported to be revoking California's Clean Air Act exemption. Trump tweeted on September 18, 2019 that he was doing so. Since 1970, California has restricted automobile emissions more than most other states.

13 other states have established automobile emission standards that are effectively, "You can sell a new car in this state if, and only if, California says it is okay to sell the car in California." In 2017, Green Car Reports listed the 13 states as Connecticut, Delaware, Maine, Maryland, Massachusetts, New Jersey, New Mexico, New York, Oregon, Pennsylvania, Rhode Island, Vermont, and Washington. Washington, D.C. also shared this policy.

If the Trump administration is successful in voiding California's power to establish stricter standards for automobile emissions and fuel economy, will this change automatically force the other 13 states to follow the federal standards?

Answer 1

If the Trump administration is successful in voiding California's power to establish stricter standards for automobile emissions and fuel economy, will this change automatically force the other 13 states to follow the federal standards?

Yes, a least that appears to be the case. There are two sections in the United States Code that apply; but one (section 177) depends on the other (section 209). If the waivers granted under section 209(b) are revoked, there will be no distinct California standard for section 177. This means that only the current Federal standard will apply.

Under Section 177, states may be approved to use the California emissions standard.

§7507. New motor vehicle emission standards in nonattainment areas
Notwithstanding section 7543(a) of this title, any State which has plan provisions approved under this part may adopt and enforce for any model year standards relating to control of emissions from new motor vehicles or new motor vehicle engines and take such other actions as are referred to in section 7543(a) of this title respecting such vehicles if—

(1) such standards are identical to the California standards for which a waiver has been granted for such model year, and
(2) California and such State adopt such standards at least two years before commencement of such model year (as determined by regulations of the Administrator).

Section 209 contains the authority for the Administrator to waive compliance with the Federal standard, which was done for California.

§7543. State standards
...
(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.