SB478 declares that the federal government has no constitutional authority to regulate diesel engines operating solely within Arkansas. The act asserts the state's 10th Amendment-protected rights, prohibiting any state agency or local government from enforcing federal rules requiring the use of diesel exhaust fluid or other federal emissions mandates on intrastate engines. It allows for the manufacture, sale, and use of diesel engines that do not require diesel exhaust fluid ("exempt engines") within Arkansas, and deems any federal regulation of such engines invalid in the state. Violations by state entities can result in civil fines up to $5,000, suspension of regulatory authority for 12 months, or termination and a five-year employment ban for individual violators.

The Arkansas State Senate passed SB478 on April 8, 2025 by a vote of 35 to 0. We have assigned pluses to the ayes because the several states have both the right and duty to interpose on behalf of their citizens and nullify unconstitutional federal actions. The U.S. Constitution created a federal government of limited, enumerated powers, reserving all others to the states or the people under the 10th Amendment. When the federal government exceeds those bounds, interposition and nullification are the proper remedies. SB478 rightly defends Arkansas' sovereignty, reasserts the federal government's limits under Article I, Section 8, and upholds constitutional federalism.